Wavelength-multiplexed light amplifying apparatus,optical amplifier and optical add-and-drop apparatus using wavelength-multiplexed light amplifying basic unit

ABSTRACT

The present invention relates to a wavelength-multiplexed light amplifying apparatus to be used in a state of being connected to a branching portion of a multiplexing/branching unit or to be used for a m×n matrix optical switch of an optical cross-connect. The wavelength-multiplexed light amplifying apparatus is composed of a plurality of wavelength-multiplexed light amplifying units and an optical feedback loop system. A signal input port of an optical branching section in one wavelength-multiplexed light amplifying unit is connected to an output side of an optical amplifying section in the preceding wavelength-multiplexed light amplifying unit. This configuration disperses the power loss of an optical signal to perform amplification with high efficiency and further enables the expansion of the branching port in the in-service. Additionally, it can eliminate the need for the installation of an optical isolator or reduce the required value of isolation to stabilize the system, or can accomplish the control to maintain the gains of all ports constant.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a Divisional Application of U.S. patent application Ser. No. 09/632/588, filed Aug. 4, 2000, now allowed, which is a Continuation-In-Part (CIP) of U.S. patent application Ser. No. 09/490,845, filed Jan. 24, 2000, now U.S. Pat. No. 6,426,832, the disclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a wavelength-multiplexed light amplifying apparatus, an optical amplifier, and further to an optical add-and-drop apparatus (optical ADM:Optical Add-Drop Multiplexer) using a wavelength-multiplexed light amplifying basic unit.

[0004] 2. Description of the Related Art

[0005] In recent years, the improvement of the transmission technology using an optical fiber has developed enterprises related to point-to-point wavelength division multiplexing (WDM), where the technical development for a photonic network has taken place extensively. This photonic network signifies a network which uses optical wavelengths as identification information for multiplexing and non-multiplexing. As one of the features the photonic network has, there is an optical add-and-drop function. The dropping function of an optical add-and-drop unit in this photonic network is realizable by a broadcast function. This broadcast function means a function of distributing the directions multiplexed signals advance in, but not different high output level in an optical amplifying system, the loss of the optical signal in the latter section of the EDF optical amplifier 71 a has great influence on the efficiency. This efficiency signifies the efficiency of conversion from an excitation optical power into a signal optical power.

[0006] In FIG. 18, let it be assumed that the dropped 32ch-wavelength-multiplexed signal is distributed or branched into 16 ports. Additionally, let it be assumed that 0 dBm per Ich (which will be referred to hereinafter as 0 dBm/ch) is kept as an optical signal quality at each branch (division) port. In this case, for maintaining 0 dBm/ch at the optical signal quality at each branch port, an output power at each branch port requires 15 dBm (32 mW for 32ch), and a value forming the sum of 15 dBm and a theoretical limit value on the division into 16 becomes necessary at an output terminal of the EDF optical amplifier 71 a. This theoretical limit value signifies the value of the original optical signal needed for the optical signal to have a given quality after the division. As well known, when the original optical signal is divided into two, each of the optical signals after the division results in the loss of 3 dB as compared with the original value, and when being divided into 16, each of the optical signals after the division suffers a loss of 12 dB as compared with the original value. Accordingly, when one optical signal is divided into 16, the original optical signal is required to have a value larger by at least 12 dB than the power value of each of the divided optical signals. In other words, in the case that one optical signal is divided into 16, the theoretical limit value of the output power of the EDF optical amplifier 71 a becomes 12 dB, and in this instance, the needed value at the output terminal of the EDF optical amplifier 71 a comes to 15 dBm+12 dBm=27 dBm. Additionally, taking into consideration an excess loss of the distribution (branch) optical coupler 71 b, the output power of (27+α) dBm becomes necessary at the output terminal of the EDF optical amplifier 71 a. In the following description, this value a will be taken as 2 dB, for example. That is, the output power of 29 dBm becomes necessary at the output terminal of the EDF optical amplifier 71 a.

[0007] On the other hand, looking at a system of input to this EDF optical amplifier 71 a, since 0 dBm/ch is needed while 32ch is dropped, the total input power to the EDF optical amplifier 71 a comes to 15 dBm. Besides, this EDF optical amplifier 71 a is made up of optical parts such as an optical isolator and an optical coupler, which causes a loss of approximately 2 dB. In consequence, a large output of 31 dBm (=27+2+2=1260 mW) develops at the output terminal of an EDFA (Erbium-Doped Fiber Amplifier) (not shown) in the EDF optical amplifier 71 a, while the output terminal of the EDF optical amplifier 71 a produces 29 dBm (=790 mW). Additionally, assuming that the conversion efficiency from an excitation light to a signal light in the EDF optical amplifier 71 a is 50%, the required excitation power reaches 1580 mW.

[0008] Thus, the operating conditions of this optical amplifying section 71 a are as follows.

[0009] 1) input power: 15 dBm (0 dBm/ch, 32 waves)

[0010] 2) output power: 29 dBm (31 dBm at the output terminal of EDFA)

[0011] 3) gain: 14 dB

[0012] 4) required excitation power: 1580 mW

[0013] From this, it is found that a large signal optical power of 1260 mW×0.37=470 mW (1260 mW=31 dBm) disappears between the output terminal of the EDF optical amplifier 71 a and the output terminal of the optical amplifying section 71 a. That is, since an excess loss (0.37; corresponding to −2 dB) occurs at the time when the signal power rises, the useless signal power increases. Additionally, even if the divisions is small in number, because a high-output optical amplifier becomes necessary at the initial introduction, the initial investment becomes higher.

[0014] For this reason, there is a need to realize the broadcast function efficiently. That is, if a broadcast system with higher efficiency is realized, a signal with a desired wavelength becomes selectable in a manner that an optical filter for performing wavelength selection at every port is placed at a spot subsequent to the point of distribution of the optical signal, and the flexible distribution becomes feasible. Besides, if realized, this broadcast function is also applicable to a distribution system such as an optical subscriber system.

[0015] In addition, Japanese Laid-Open (Kokai) HEI 3-123323 (which will be referred to hereinafter as a publication) discloses a technique on a fiber coupler for preventing the reduction of the optical intensity, occurring whenever the division takes place, to considerably increase the dividable hierarchies. FIG. 19 shows an arrangement of this fiber coupler. In the arrangement shown in FIG. 19, fiber couplers are combined to provide three division hierarchies, where a signal light S_(o) inputted to an input section 3 is introduced into a branching section 2 while an excitation light P inputted to another input section 3 is also led to the branching section 2 so that they are distributed to an output sides 4 of an optical fiber and then transmitted to amplification fibers 5. The signal light amplified in the amplification fiber 5 is subsequently distributed through a second branching section 12 to second optical fibers 13 and further distributed through third branching sections 14 to third optical fibers 15, thus outputting signal lights S₁ to S₈ therefrom.

[0016] However, the technique disclosed by this publication relates to light with a single wavelength, but the publication does not refer to a technique on wavelength-multiplexed light at all.

SUMMARY OF THE INVENTION

[0017] The present invention has been developed with a view to eliminating the above-mentioned problems, and it is therefore an object of the invention to provide a wavelength-multiplexed light amplifying apparatus and an optical amplifier to be connected to branch ports of an add-and-drop apparatus for adding a wavelength multiplexed optical signal and for dropping and outputting a wavelength multiplexed optical signal, or a wavelength-multiplexed light amplifying apparatus and an optical amplifier to be used for an m×n matrix optical switch in an optical cross-connect unit, where basic amplifying units to which 1:1 optical couplers and EDFAs are coupled alternately are connected in a multistage fashion to distribute a power loss of an optical signal for accomplishing a high-efficiency amplification, to enable the expansion of the branch ports on the in-service, to render an optical isolator unnecessary or reduce the required isolation value for stabilizing the system, and to implement control so that the gains at all the ports are maintained constant, and further to provide an optical add-and-drop apparatus using a wavelength-multiplexed light amplifying basic unit.

[0018] For this purpose, in accordance with this invention, there is provided a wavelength-multiplexed light amplifying apparatus comprising a plurality of wavelength-multiplexed light amplifying units, with a signal input port of an optical branching section in one wavelength-multiplexed light amplifying unit connected to an output side of an optical amplifying section in the preceding wavelength-multiplexed light amplifying unit, and an optical feedback loop system.

[0019] Thus, as compared with the case of amplifying a wavelength-multiplexed optical signal as a whole, the optical signal power reduces considerably, which presents a higher-efficiency characteristic. In addition, since each of the first to third wavelength-multiplexed light amplifying basic units is composed of the optical branching section having the two input ports and the two output ports for outputting a wavelength-multiplexed optical signal through the two output ports and the optical amplifying sections connected to the two output ports of this optical branching section, the amplification mediums are disposed dispersedly in an optical distribution system, which permits the realization of a stable optical amplifier without relying on an optical isolator. Still additionally, since the optical amplifying sections and the optical branching sections are connected alternately to each other, the effective gain in the amplifier lowers to produce a reflection resistance, and if the gain of the optical amplifying section is set at a lower value than the division (branch) loss, the effective gain becomes minus, which eliminates the need for each optical amplifying section being interposed between optical isolators and, hence, simplifies the system. Moreover, the excess loss of a system including the optical amplifier and the distribution system is reducible so that the unstable operation due to the reflection is eliminable without using an optical isolator.

[0020] In this configuration, it is also appropriate that an auxiliary excitation light source is connected to the other input port of at least one of the plurality of wavelength-multiplexed light amplifying basic units, or that an isolator is connected to the signal input port of the first wavelength-multiplexed light amplifying basic unit in at least one of the plurality of wavelength-multiplexed light amplifying units.

[0021] Furthermore, it is also appropriate that an auxiliary excitation light source is connected to the other input port of the aforesaid wavelength-multiplexed light amplifying basic unit, or that an isolator is connected to the signal input port of the aforesaid wavelength-multiplexed light amplifying basic unit.

[0022] Still furthermore, it is also appropriate that an auxiliary excitation light source is connected to the other input port in at least one of the plurality of wavelength-multiplexed light amplifying basic units, or that an isolator is connected to the signal input port in at least one of the plurality of wavelength-multiplexed light amplifying basic units.

[0023] In this case, it is possible to strengthen the excitation optical power when needed, and since the excitation light source can be distributed equally through the use of the optical branching section (1:1 optical coupler), the excitation optical power can be supplied equally through a simple configuration to each of the optical amplifying sections, which eliminates the need for the use of multiplexing units for adding or multiplexing an excitation light and a signal light, or reduces the required number of multiplexing units.

[0024] In addition, in addition to avoiding the unstable operation due to the reflecting and returning light, the reverse ASE gathering from the output side to the input side is shut off, thereby preventing the saturation of the optical amplifier due to the reverse ASE.

[0025] Moreover, it is also acceptable that, in at least two of the plurality of wavelength-multiplexed light amplifying units, the output side of the optical amplifying section of each of the second and third wavelength-multiplexed light amplifying basic units in the preceding wavelength-multiplexed light amplifying unit are connected through an oblique (skew) connector to the input side of the signal input port of the first wavelength-multiplexed light amplifying basic unit in the subsequent-stage wavelength-multiplexed light amplifying unit.

[0026] Furthermore, it is also acceptable that, in at least two of the plurality of wavelength-multiplexed light amplifying basic units, the output side of the optical amplifying section of the preceding wavelength-multiplexed light amplifying basic unit is connected through an oblique connector to the input side of the signal input port in the subsequent-stage wavelength-multiplexed light amplifying basic unit.

[0027] In this case, light reflection does not occur on the connector surface, so that useless light amplification is preventable.

[0028] Still furthermore, it is also possible that an input monitor is provided to detect the input to the fourth wavelength-multiplexed light amplifying basic unit, and the gain control means includes a control section for producing a gain control signal on the basis of the output information and the detection result by the input monitor and for supplying the gain control signal to an excitation light source provided on the input side of the gain equalizer for controlling an excitation state of the excitation light source.

[0029] In this case, the control of the wavelength characteristic of the gain at all the branch ports becomes feasible, and the control of the wavelength characteristic of the gain becomes possible at all the ports in a manner that feedback control is implemented for the forefront excitation light source.

[0030] Furthermore, in accordance with this invention, there is provided a wavelength-multiplexed light amplifying apparatus comprising a plurality of wavelength-multiplexed light amplifying basic units, with the wavelength-multiplexed light amplifying basic units being connected in a multistage fashion in a manner that the signal input port of the optical branching section is connected to an output side of the optical amplifying section of the preceding wavelength-multiplexed light amplifying basic unit and an optical feedback loop system.

[0031] Thus, if the user wants the extension of the service, the extension in service becomes easily feasible. That is, by newly and additionally buying this wavelength-multiplexed light amplifying basic unit, the user can easily increase the number of divisions. This extension can be performed without interrupting the current service. In addition, if the forefront excitation light source has a power sufficient to excite the rearmost wavelength-multiplexed light amplifying basic unit, there is no need to place an excitation light source in the intermediate wavelength-multiplexed light amplifying basic units. Still additionally, an excitation light source can be installed in the intermediate wavelength-multiplexed light amplifying basic units when needed, and excitation light sources can be connected to all the wavelength-multiplexed light amplifying basic units, thus achieving flexible excitation. Besides, the respective elements are constructed with optical parts having the same characteristic, and this suits the mass production, and offers excellent expansion.

[0032] Furthermore, in this configuration, it is also possible that an input monitor is provided to detect the input to the forefront unit of the plurality of wavelength-multiplexed light amplifying basic units arranged in multistage fashion, and the gain control means includes a control section for producing a gain control signal on the basis of the output information and the detection result by the input monitor and for supplying the gain control signal to an excitation light source provided on the input side of the gain equalizer for controlling an exciting state of the excitation light source.

[0033] In this case, the control of the wavelength characteristic of the gain at all the branch ports becomes feasible, and the control of the wavelength characteristic of the gain becomes possible at all the ports in a manner that feedback control is implemented for the forefront excitation light source.

[0034] Still furthermore, in accordance with this invention, there is provided a wavelength-multiplexed light amplifying apparatus comprising a wavelength-multiplexed light amplifying basic unit and an optical feedback loop system.

[0035] Thus, this system can be offered as simple equipment to the users, while the users can easily increase the number of divisions without interrupting the current service.

[0036] In this configuration, it is also possible that an input monitor is provided to detect the input to the wavelength-multiplexed light amplifying basic unit arranged in multistage fashion, and the gain control means includes a control section for producing a gain control signal on the basis of the output information and the detection result by the input monitor and for supplying the gain control signal to an excitation light source provided on the input side of the gain equalizer for controlling an exciting state of the excitation light source.

[0037] Thus, the control of the wavelength characteristic of the gain at all the branch ports becomes feasible, and the control of the wavelength characteristic of the gain becomes possible at all the ports in a manner that feedback control is implemented for the forefront excitation light source.

[0038] In this case, it is possible that the optical branching section of the aforesaid wavelength-multiplexed light amplifying basic unit is constructed as a 1:1 optical coupler and the optical amplifying section of the aforesaid wavelength-multiplexed light amplifying basic unit is constructed to have a gain equivalent to a division loss in the optical branching section or a gain lower than the division loss, or that the optical branching section, the optical amplifying section and a connecting section between the optical branching section and optical amplifying section in the aforesaid wavelength-multiplexed light amplifying basic unit are constructed as an optical waveguide made from a glass or a semiconductor.

[0039] Thus, the division and the amplification are alternately conducted when viewed from the output side, and each of the optical amplifying sections can maintain the theoretical limit value (for example, 15 dBm) as long as it performs the amplification at a gain (for example, 18 dB) with a level or magnitude capable of compensating for the power loss (for example 3 dB) occurring at the division into two, thereby presenting high-efficiency expansion. Additionally, a stable optical amplifier becomes realizable, and there occurs no situation where the optical power strengthens locally, which offers a higher-efficiency characteristic.

[0040] In addition, it is also possible that the aforesaid gain control means is equipped with a filter for selecting a desired-wavelength optical signal from the output information and an optical attenuator for attenuating the desired-wavelength optical signal extracted through the filter for the laser resonance and further for supplying the attenuated optical signal to the input side of the gain equalizer. This filter can also be constructed as a filter for selecting an optical signal with a wavelength being out of a wavelength band needed for transmission.

[0041] Thus, the port to be feedbacked is only one in number, and the forefront excitation light source can vary the excitation light to maintain the gain of each of the ports constant, so that the gains of all the ports are maintainable constant with a simple construction.

[0042] Still furthermore, in accordance with this invention, there is provided a wavelength-multiplexed light amplifying apparatus made to be connected to a branching port of a multiplexing/branching unit which receives a wavelength-multiplexed optical signal composed from optical signals of M kinds of wavelengths (M signifies a natural number) and a wavelength-multiplexed optical signal composed from optical signals of N kinds of wavelengths and further which branches them into a wavelength-multiplexed optical signal composed from optical signals of (M+N−L) kinds of wavelengths (N and L signify a natural number) and a wavelength-multiplexed optical signal composed from optical signals of L kinds of wavelengths, the wavelength-multiplexed light amplifying apparatus comprising a plurality of wavelength-multiplexed light amplifying basic units each including an optical branching section for receiving the wavelength-multiplexed optical signal, and optical amplifying sections respectively connected to the two output ports of the optical branching section, with the wavelength-multiplexed light amplifying basic units being connected in a multistage fashion in a manner that the signal input port of the optical branching section in one wavelength-multiplexed basic unit is connected to an output side of the optical amplifying section of another preceding wavelength-multiplexed light amplifying basic unit, and an excitation light source connected to the other input port of the optical branching section of the forefront unit of the plurality of wavelength-multiplexed light amplifying basic units.

[0043] Thus, a broadcast function is easily realizable, and the extension of the branch ports on the in-service becomes possible.

[0044] Moreover, in accordance with this invention, there is provided a wavelength-multiplexed light amplifying apparatus for use in an m×n matrix optical switch (m and n signify a natural number) in an optical cross-connect unit, comprising a plurality of wavelength-multiplexed light amplifying basic units each including an optical branching section for receiving a wavelength-multiplexed optical signal and optical amplifying sections respectively connected to the two output ports of the optical branching section, with the wavelength-multiplexed light amplifying basic units being connected in a multistage fashion in a manner that the signal input port of the optical branching section in one wavelength-multiplexed basic unit is connected to an output side of the optical amplifying section of another preceding wavelength-multiplexed light amplifying basic unit, and an excitation light source connected to the other input port of the optical branching section of the forefront unit of the plurality of wavelength-multiplexed light amplifying basic units.

[0045] Thus, this system is applicable to a distribution system such as an optical subscriber system, and the expansion on the in-service is easily feasible.

[0046] In addition, in accordance with this invention, there is provided an optical add-and-drop apparatus using wavelength-multiplexed light amplifying basic units, comprising a first-stage unit for distributing a wavelength-multiplexed optical signal composed from optical signals of X kinds of wavelengths, a first broadcast unit connected to an output side of the first-stage unit for broadcasting the wavelength-multiplexed optical signal outputted from the first-stage unit, a second broadcast unit connected to the output side of the first-stage unit for broadcasting the wavelength-multiplexed optical signal outputted from the first-stage unit and for dropping an optical signal, an optical carrier signal outputting section for outputting an optical carrier signal and for adding an optical signal and a selecting section connected to an output side of the first broadcast unit and an output side of the optical carrier signal outputting section for selectively outputting the wavelength-multiplexed optical signal from the first broadcast unit and the optical carrier signal from the optical carrier signal outputting section, wherein the first-stage unit is constructed as a first-stage wavelength-multiplexed light amplifying basic unit composed of an optical branching section for receiving the wavelength-multiplexed optical signal, composed from the optical signals of the X kinds of wavelengths and optical amplifying sections respectively connected to the two output ports of the optical branching section, and the first broadcast unit includes a plurality of multiplexing wavelength-multiplexed light amplifying basic units each identical in configuration to the first-stage wavelength-multiplexed light amplifying basic unit, with these multiplexing wavelength-multiplexed light amplifying basic units being connected in a multistage fashion and the second broadcast unit includes a plurality of distribution wavelength-multiplexed light amplifying basic units each identical in configuration to the first-stage wavelength-multiplexed light amplifying basic unit, with the distribution wavelength-multiplexed light amplifying basic units being connected in a multistage fashion and further includes a plurality of dropping optical filters for selectively outputting an optical signal with a predetermined wavelength to an output side of the optical amplifying section, while the selecting section includes switch sections, Y in number, for conducting switching between passage and interruption of the wavelength-multiplexed optical signal, coupling sections, Y in number, for conducting coupling between a wavelength-multiplexed optical signal line of the switching sections and an optical carrier signal line from the optical carrier signal outputting section and a multiplexing section for wavelength-multiplexing Y kinds of optical signals from the coupling sections to output a wavelength-multiplexed optical signal, and the switching sections are made to selectively conduct switching between an operation of allowing Y kinds of wavelength-multiplexed optical signals from the first broadcast unit to pass to be outputted from the multiplexing section and an operation of allowing Y kinds of optical carrier signals from the optical carrier signal outputting section to pass to be outputted from the multiplexing section.

[0047] Thus, the add-and-drop function is realizable with the same wavelength-multiplexed light amplifying units, which permits flexible extension and expansion of an optical network.

[0048] Moreover, in the wavelength-multiplexed light amplifying apparatus, it is also appropriate that an excitation light re-inputting section is provided in connection relation between the fourth wavelength-multiplexed light amplifying basic unit and at least the forefront wavelength-multiplexed light amplifying unit for removing a wavelength-multiplexed optical signal component from a leakage optical signal outputted from the fourth wavelength-multiplexed light amplifying basic unit to extract an excitation light and further for branching the extracted excitation light into a plurality of excitation lights to output the plurality of excitation lights, that an excitation light re-inputting section is provided between the forefront unit of the wavelength-multiplexed light amplifying base units connected in a multistage fashion and at least the forefront wavelength-multiplexed light amplifying unit for removing a wavelength-multiplexed optical signal component from a leakage optical signal outputted from the forefront wavelength-multiplexed light amplifying basic unit to extract an excitation light and further for branching the extracted excitation light into a plurality of excitation lights to output the plurality of excitation lights, or that provided are a fourth wavelength-multiplexed light amplifying basic unit identical in configuration to the wavelength-multiplexed light amplifying basic unit, with an output side of one optical amplifying section being connected to the signal input port of the wavelength-multiplexed light amplifying unit and an excitation light re-inputting section provided between the fourth wavelength-multiplexed light amplifying basic unit and the wavelength-multiplexed light amplifying basic unit for removing a wavelength-multiplexed optical signal component from a leakage optical signal outputted from the fourth wavelength-multiplexed light amplifying basic unit to extract an excitation light and further for branching the extracted excitation light into a plurality of excitation lights to output the plurality of excitation lights.

[0049] In addition, it is also appropriate that the excitation light re-inputting section includes a filter for removing a wavelength-multiplexed optical signal component from the leakage optical signal outputted from the fourth wavelength-multiplexed light amplifying basic unit to extract an excitation light and an optical branching section which is connected to the filter and an output side of which is connected to at least the signal input port of the second wavelength-multiplexed light amplifying basic unit in the forefront wavelength-multiplexed light amplifying unit and the signal input port of the third wavelength-multiplexed light amplifying basic unit, for putting the excitation light extracted by the filter in the forefront wavelength-multiplexed light amplifying unit, that the excitation light re-inputting section includes a filter for removing a wavelength-multiplexed optical signal component from the leakage optical signal outputted from the forefront wavelength-multiplexed light amplifying basic unit to extract an excitation light and an optical branching section which is connected to the filter and an output side of which is connected to at least the signal input port of the second wavelength-multiplexed light amplifying basic unit in the forefront wavelength-multiplexed light amplifying unit and the signal input port of the third wavelength-multiplexed light amplifying basic unit, for putting the excitation light extracted by the filter in the forefront wavelength-multiplexed light amplifying unit, or that the excitation light re-inputting section includes a filter for removing the wavelength-multiplexed optical signal component from a leakage optical signal outputted from the fourth wavelength-multiplexed light amplifying basic unit to extract an excitation light and an optical branching section which is connected to the filter and an output side of which is connected to the other input port of the wavelength-multiplexed light amplifying unit, for putting the excitation light extracted by the filter in the wavelength-multiplexed light amplifying basic unit.

[0050] Furthermore, in accordance with this invention, an optical add-and-drop apparatus using wavelength-multiplexed light amplifying basic units comprises a first stage unit for broadcasting a first wavelength-multiplexed optical signal composed from optical signals of X kinds of wavelengths, a first broadcast unit connected to an output side of the first-stage unit for broadcasting the first wavelength-multiplexed optical signal outputted from the first-stage unit, an additional signal outputting section for multiplexing a plurality of optical carrier signals to output a second wavelength-multiplexed optical signal, and a transmission outputting section connected to an output side of the first-stage unit and an output side of the additional signal outputting section for coupling the first wavelength-multiplexed optical signal from the first-stage unit with the second wavelength-multiplexed optical signal from the additional signal outputting section, wherein the first-stage unit is constructed as a first-stage wavelength-multiplexed light amplifying basic unit composed of an optical branching section having two input ports and two output ports for receiving the wavelength-multiplexed optical signal, composed from the optical signals of X kinds of wavelengths, through a signal input port forming one of the two input ports to output the first wavelength-multiplexed optical signal through the two output ports, with the other signal input port being connected to an excitation light source and optical amplifying sections respectively connected to the two output ports of the optical branching section, and the first broadcast unit includes a plurality of branching wavelength-multiplexed light amplifying basic units each identical in configuration to the first-stage wavelength-multiplexed light amplifying basic unit, with the plurality of branching wavelength-multiplexed light amplifying basic units being connected in a multistage fashion in a manner that the signal input port of the optical branching section in one multiplexing wavelength-multiplexed light amplifying basic unit is connected to an output side of the optical amplifying section in the preceding branching wavelength-multiplexed light amplifying basic unit, with one input port of the optical branching section in the forefront unit of the plurality of branching wavelength-multiplexed light amplifying basic units being connected to an output port of one optical amplifying section in the first-stage wavelength-multiplexed light amplifying basic unit, and with a plurality of dropping optical filters each for selecting the optical signal having a predetermined wavelength being provided at an output side of the optical amplifying section of the rearmost basic unit of the plurality of branching wavelength-multiplexed light amplifying basic units, and further the additional signal outputting section includes a plurality of optical carrier signal generating sections each for outputting an optical carrier signal and a multiplexing section connected to the side of the plurality of optical carrier signal generating sections for multiplexing the plurality of optical carrier signals from the plurality of optical carrier signal outputting sections to output the second wavelength-multiplexed optical signal, and even the transmission outputting section includes a coupling section connected to the first-stage unit and further to the additional signal outputting section for coupling the first wavelength-multiplexed optical signal from the first-stage unit with the second wavelength-multiplexed optical signal from the additional signal outputting section to output a third wavelength-multiplexed optical signal and an optical amplifier for amplifying the third wavelength-multiplexed optical signal from the coupling section to output the amplified third wavelength-multiplexed optical signal.

[0051] In addition, in the optical add-and-drop apparatus, it is also possible that further provided are an amplifying section provided on an input side of the first-stage unit for amplifying an optical signal to output an amplified optical signal and first dispersion compensating section connected to the amplifying section and further to the signal input port of the optical branching section in the first-stage unit for compensating for dispersion of the amplified optical signal to put a compensation optical signal subjected to the dispersion compensation in the signal port of the optical branching section.

[0052] Still additionally, in the optical add-and-drop apparatus, it is also possible that further provided is a second dispersion compensating section connected to the optical amplifier of the transmission outputting section for compensating for dispersion of the amplified third wavelength-multiplexed optical signal to put a compensation optical signal subjected to the dispersion compensation.

[0053] Moreover, in accordance with this invention, there is provided a wavelength-multiplexed light amplifying apparatus comprising an optical amplifying medium for amplifying input light, an optical branching/amplifying section for branching the optical signal, outputted from the optical amplifying medium, into a plurality of branched optical signals and further for amplifying each of the branched optical signals for compensating for a loss resulting from the branching, and gain control means connected to the optical branching/amplifying section for controlling a gain of the optical amplifying medium on the basis of the optical signal from the optical branching/amplifying section.

[0054] Thus, this method can conduct extraction processing for a desired wavelength in inconsideration of the influence on the other wavelengths and further can avoid, for example, the occurrence of coherent crosstalk easily. Additionally, leakage excitation light can be reused, thereby accomplishing efficient amplification.

[0055] Further, in accordance with this invention, there is provided an optical amplifier comprising a beam splitter splitting an input light, having a portion for amplifying

[0056] The input light, a pumping light source emitting a pumping light, a monitor monitoring an output light of the beam splitter, and a controller controlling a power of the pumping light in accordance with an output of the monitor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0057]FIG. 1 is an illustration of a wavelength-multiplexed light amplifying apparatus according to a first embodiment of the present invention;

[0058]FIG. 2 is a block diagram illustrating an aspect of the present invention;

[0059]FIG. 3 is a block diagram showing a configuration of a wavelength-multiplexed light amplifying unit in the first embodiment of this invention;

[0060]FIG. 4 is an illustration of a configuration of an optical branching section in the first embodiment of this invention;

[0061]FIG. 5 is a block diagram showing a configuration of another wavelength-multiplexed light amplifying unit in the first embodiment of this invention;

[0062]FIG. 6 is an illustration of a configuration of a block of a further wavelength-multiplexed light amplifying unit in the first embodiment of this invention;

[0063]FIG. 7 is an illustration of another wavelength-multiplexed light amplifying apparatus according to the first embodiment of this invention;

[0064]FIG. 8 is an illustration of a wavelength-multiplexed light amplifying apparatus according to a first modification of the first embodiment of this invention;

[0065]FIG. 9 is a block diagram showing a configuration of a wavelength-multiplexed light amplifying unit in the first modification of the first embodiment of this invention;

[0066]FIG. 10 is an illustration of a wavelength-multiplexed light amplifying apparatus according to a second modification of the first embodiment of this invention;

[0067]FIG. 11 is an illustration of a wavelength-multiplexed light amplifying apparatus according to a second embodiment of this invention;

[0068]FIG. 12 is an illustration of one example of configuration of a gain control means in the second embodiment of this invention;

[0069]FIG. 13 is an illustration of another wavelength-multiplexed light amplifying apparatus according to the second embodiment of this invention;

[0070]FIG. 14 is a block diagram schematically showing an optical cross-connect unit to which this invention is applicable;

[0071]FIG. 15 is a block diagram schematically showing an optical add-and-drop apparatus to which this invention is applicable;

[0072]FIG. 16 is an illustration of a different wavelength-multiplexed light amplifying apparatus according to a first modification of the first embodiment of this invention;

[0073]FIG. 17 is a block diagram showing a further optical add-and-drop apparatus to which this invention is applicable;

[0074]FIG. 18 is an illustration of one example of configuration of an optical add-and-drop multiplexing apparatus; and

[0075]FIG. 19 is an illustration of a configuration of a fiber coupler.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0076] Embodiments of the present invention will be described hereinbelow with reference to the drawings.

[0077] (A) Description of a First Embodiment of the Invention

[0078]FIG. 1 shows a wavelength-multiplexed light amplifying apparatus according to a first embodiment of this invention. A wavelength-multiplexed light amplifying apparatus, designated generally at numeral 25 in FIG. 1, is for distributing one optical signal to 32 users, and employs optical feedback control. Besides, in the wavelength-multiplexed light amplifying apparatus 25 shown in FIG. 1, of the 32 users, 16 users are put in the illustration, and this system is mainly for use in a system shown in FIG. 2.

[0079]FIG. 2 is a block diagram illustrating an aspect of the optical add-and-drop apparatus (optical add-and-drop apparatus using wavelength-multiplexed light amplifying basic unit) and shows a configuration of a broadcast section based upon the first embodiment of this invention. In the optical add-and-drop apparatus 20, as shown in FIG. 2, ch1 to ch64 are dropped from an optical signal transmitted from the left side of FIG. 2 and wherein 64 wavelengths (ch1 to ch64) are multiplexed, while different ch65 to ch96 are added to the optical signal so that an optical signal wherein 96 wavelengths (ch1 to ch96) are multiplexed is transmitted toward the right side of FIG. 2. This optical add-and-drop apparatus 20 comprises optical amplifiers 20 a and 20 c, an optical coupler 20 b, 20 b′, an optical branching section 21 and an optical multiplexing section 22. The optical multiplexing section 22 is composed of an EDF optical amplifier 22 a for conducting optical amplification, a multiplexing section 22 b for multiplexing or coupling a plurality of optical signals, an one-wave optical amplifier 22 c for amplifying one wave, a variable wavelength tunable laser 22 d and a PD (Photo Diode) 22 e for conducting an O/E conversion, while the optical branching section 21 is composed of an EDF optical amplifier 21 a for amplifying power to maintain the original optical signal power and an optical coupler 21 b for distributing an optical signal. The function of this PD 22 e is receiving a signal and outputting a signal to be used for transmitting the same signal having another wavelength by the tunable laser 22 d. Further, the combination of the PD 22 e and the tunable laser 22 d is a transponder.

[0080] A wavelength-multiplexed optical signal coming from the left side of FIG. 2 is amplified in the optical amplifier 20 a and a portion of the optical signal is dropped in the optical coupler 20 b while an optical signal coming from the adding section 22 is added to be inputted to the optical coupler 20 b′. Then in the optical coupler 20 b′, the optical signal coming from the adding section 22 and the optical signal (ch1 to ch64) amplified in the optical amplifier 20 a are added (ch1 to ch96). The added optical signal is then amplified in the optical amplifier 20 c and transmitted toward the right side of FIG. 2.

[0081] Further, in the branching section 21, for example, the optical signal with 64 wavelengths(ch1 to ch64) multiplexed is amplified up to 64 times in the EDF optical amplifier 21 a and then demultiplexed in the optical coupler 21 b so that the 64-wavelength-multiplexed (ch1 to ch64) optical signal is distributed to each port, and an optical signal with a desired wavelength is selected through a wavelength section optical filter 23. Accordingly, owing to this optical add-and-drop apparatus 20, a wavelength-multiplexed signal is outputted to each port in a broadcasted condition. This function can be realized by using the wavelength-multiplexed light amplifying apparatus 25.

[0082] Returning again to FIG. 1, the wavelength-multiplexed light amplifying apparatus 25 comprises a plurality of wavelength-multiplexed light amplifying units 30, connected in a multistage fashion as indicated as sections (B), (C), (D), (E) and (F), and an optical feedback loop system 34 (the left side in this illustration). The relationship in connection therebetween will be described hereinbelow with reference to FIG. 3.

[0083]FIG. 3 is a block diagram showing a configuration of a wavelength-multiplexed light amplifying unit 30 in the first embodiment of this invention. In FIG. 3, the wavelength-multiplexed light amplifying unit 30 includes three basic units, a first wavelength-multiplexed light amplifying basic unit 31, a second wavelength-multiplexed light amplifying basic unit 32 and a third wavelength-multiplexed light amplifying basic unit 33, with an optical isolator 3 being connected to one signal input port of the first wavelength-multiplexed light amplifying basic unit 31 while a excitation light source 4 being connected to the other signal input port thereof.

[0084] In this configuration, the first wavelength-multiplexed light amplifying basic unit 31 is composed of an optical branching section 1 a and optical amplifying sections 2 a and 2 b. The optical branching section 1 a has two input ports P₁, P₂ and two output ports P₃, P₄, and receives a wavelength-multiplexed optical signal, composed from optical signals with a plurality of wavelengths, through its signal input port P₂ forming one of the two input ports P₁ and P₂ to output the wavelength-multiplexed optical signal through the two output ports P₃ and P₄. Besides, this optical branching section 1 a is constructed using a 1:1 optical coupler. Furthermore, the optical amplifying sections 2 a and 2 b are connected to the two output ports P₃ and P₄ of the optical branching section 1 a, respectively, and are constructed as optical amplifying sections having a gain equivalent to a division (branch) loss in the optical branching section 1 a or lower than the branch loss. Additionally, each of these optical amplifying sections 2 a and 2 b is constructed using an EDF (Erbium-Doped Fiber) or EDW (Erbium-doped Waveguide), and has a gain of approximately 3 to 6 dB close to the division loss (3 dB).

[0085] The second wavelength-multiplexed light amplifying basic unit 32 has the same arrangement as that of the first wavelength-multiplexed light amplifying basic unit 31, where the input side of its signal input port P₂ is connected to the output side of the one optical amplifying section 2 a of the first wavelength-multiplexed light amplifying basic unit 31. Furthermore, the third wavelength-multiplexed light amplifying basic unit 33 similarly has the same arrangement as that of the first wavelength-multiplexed light amplifying basic unit 31, where the input side of its signal input port P₁ is connected to the output side of the other optical amplifying section 2 b of the first wavelength-multiplexed light amplifying basic unit 31. In this case, the names of the input/output ports of these optical branching sections 1 b and 1 c are identical to those of the input/output ports of the optical branching section 1 a, and in the following description, these port names will sometimes be used although being omitted from the illustrations.

[0086] Furthermore, in the wavelength-multiplexed light amplifying unit 30, the optical isolator 3 is an optical part which allows the transmission of a light beam in the forward direction but intercepting a light bean in the reverse direction, and displays a high resistance to reflecting and returning light. The excitation light source 4 is an LD (Laser Diode) controllable from the external, and is made such that its output is controllable variably. In addition, in this wavelength-multiplexed light amplifying unit 30, connectors 5 are installed on the input side of the optical isolator 3, the input port side of the optical branching section 1 b, the input port side of the optical branching section 1 c and the output sides of the optical amplifying sections 2 c, 2 d, 2 e and 2 f. Light from an auxiliary excitation light source 4 a is inputted to the input port P₁ of the optical branching section 1 b and the input port P₂ of the optical branching section 1 c, so that the auxiliary excitation light source 4 a is connected to the other input port P₁ in the second wavelength-multiplexed light amplifying basic unit 32 and further to the other input port P₂ in the third wavelength-multiplexed light amplifying basic unit 33. In the illustrations referred to for the following description, the connectors 5 connected to the optical branching sections 1 b and 1 c in this wavelength-multiplexed light amplifying unit 30 will sometimes be omitted for convenience only.

[0087] As described above, in the first wavelength-multiplexed light amplifying basic unit 31, of the input side two ports P₁ and P₂ of the optical branching section 1 a, one port P₁ is used as an excitation port while the other port P₂ is used as an input port, and amplification mediums (optical amplifying sections 2 a and 2 b) having an equal gain for substantially compensating for the division loss, the excess loss of the optical coupler (optical branching section 1 a) and the connector loss are connected to the output side two ports P₃ and P₄ of the same optical branching section 1 a. In addition, the optical branching section 1 a (1:1 optical coupler) is employed simultaneously for the division (branch) of the signal optical power into two and the division of the excitation optical power into two. Furthermore, the second wavelength-multiplexed light amplifying basic unit 32 and the third wavelength-multiplexed light amplifying basic unit 33 fully have the same configuration, and the optical amplifying section 2 a in the first wavelength-multiplexed light amplifying basic unit 31 is connected to the input port P₂ of the optical branching section 1 b in the second wavelength-multiplexed light amplifying basic unit 32, while the optical amplifying section 2 b in the first wavelength-multiplexed light amplifying basic unit 31 is connected to the input port P₁ of the optical branching section 1 c in the third wavelength-multiplexed light amplifying basic unit 33. Accordingly, this wavelength-multiplexed light amplifying unit 30 also functions as a distribution system for distributing an optical signal.

[0088] On the other hand, when the input side is viewed from the uppermost one (the uppermost connector on the right side in FIG. 3) of the output side connectors 5 in the wavelength-multiplexed light amplifying unit 30 shown in FIG. 3, the optical branching sections 1 a, 1 b and the optical amplifying sections 2 a, 2 c are connected alternately connected to each other. For maintaining the output level of each of the optical amplifying sections 2 a to 2 f at 15 dBm/ch (=0 dBm/ch), the gain of each of the optical amplifying sections 2 a to 2 f can take 3 dB which makes up for the power loss originating from the division into two, and in a manner that the power of an optical signal inputted to each of the optical amplifying sections 2 a to 2 f is set at 18dBm, even if a loss of 3 dB occurs due to the division, the amplification can compensate for the loss of 3 dB. In other words, if each of the optical amplifying sections 2 a to 2 f performs the amplification at a gain (for example, 18 dB) which can compensate for the power loss (for example, 3 dB) stemming from the division into two, the theoretical limit value (for example, 15 dBm) is maintainable so that the number of users is increasable without the occurrence of attenuation. Besides, without employing a configuration in which a separate optical distribution system is provided after the amplification of an optical signal to distribute the optical signal, this invention employs an arrangement in which the amplification mediums are dispersed in the optical distribution system, so that the dispersion of the loss takes place.

[0089] Furthermore, since the optical amplifying sections 2 a to 2 f (EDFA) and the optical branching sections 1 a to 1 c (1:1 optical coupler) are connected alternately to each other, the EDFAs forming the amplification mediums are disconnected in division loss; which lowers the effective gain in the amplifier to create a resistance to reflection. That is, the operation is stable even if the required value of the isolation is low, or there is no need to interpose each of the amplifying sections 2 a to 2 f between the optical isolators 3; in consequence, it is possible to remove the unstable operation occurring due to the reflection.

[0090] This will be understood from the following description. In the case of the use of the EDFA, oscillation occurs due to light reflection between optical parts connected before and after this EDFA. Accordingly, in order to prevent the occurrence of this oscillation, a need for the installation of the optical isolator 3 exists for R₁·R₂·G²<<1, where R₁ represents a reflection coefficient, R₂ designates a reflection coefficient of an optical part following the EDFA and G denotes a gain of the EDFA. In the case that parts are connected alternately in a multistage fashion like this embodiment, since the optical parts standing before and after the EDFA produce a loss, even if the optical isolator 3 does not exist, the stabilization condition of R₁·R₂·G²<<1 is satisfied, thereby providing a stable operation. In addition, if the gain of the EDFA is lower than the loss resulting from the division loss, the effective gain of a combination of an EDFA and a 1:1 optical coupler assumes a minus value and a slight loss develops; in consequence, the optical isolator 3 becomes unnecessary. In this case, in the wavelength-multiplexed light amplifying unit 30, since the optical signal power decreases little by little toward a distribution portion, the system can be simplified while the signal quality is maintainable. Besides, the excess loss of the system including the optical amplifier and the distribution system is reducible.

[0091] Referring again to FIG. 1, of the wavelength-multiplexed light amplifying units 30 denoted by (B) to (F), the optical amplifying section 2 c in (B) is connected to (C), and similarly the optical amplifying section 2 d in (B) is connected to (D), and further the optical amplifying section 2 e in (B) is connected to (E), and even the optical amplifying section 2 f in (B) is connected to (F). That is, these wavelength-multiplexed light amplifying units 30 are connected in a multistage fashion in a manner that the signal input port P₁ of the optical branching section 1 a of the first wavelength-multiplexed light amplifying basic unit 31 in one wavelength-multiplexed light amplifying unit 30 is connected to the output side of the optical amplifying section 2 c (2 d, 2 e, 2 f) of the second or third wavelength-multiplexed light amplifying basic unit 32 or 33 in the preceding wavelength-multiplexed light amplifying unit 30 (indicated by (B)). Additionally, the output of the optical amplifying section 2 c in (C) is inputted to the optical feedback loop system 34, so that the wavelength characteristic of the gain is controlled regularly to maintain the gains of all the ports constant.

[0092]FIG. 4 is an illustration of a configuration of the optical branching section 1 a (or 1 b, 1 c) in the first embodiment of this invention. In FIG. 4, the optical branching section 1 a (1 b, 1 c) is constructed with a 1:1 optical coupler, and is made to multiplex or add optical signals inputted from the port P₁ and the port P₂ and further to branch and output an optical signal to the port P₃ and the port P₄ The power W₁ of the optical signal inputted from the port P₁ is divided so that the optical signal powers W₂ and W₃ at the port P₃ and the port P₄ are halved. Additionally, also in further embodiments and modifications thereof, these optical branching sections 1 a (1 b, 1 c) have a structure shown in FIG. 4. Incidentally, the optical branching sections 1 a to 1 c, the optical amplifying sections 2 a to 2 f and the connecting sections between the optical branching section 1 a to 1 c and the optical amplifying sections 2 a to 2 f in the first to third wavelength-multiplexed light amplifying basic units 31 to 33 can also be constructed using an optical waveguide made of a glass or a semiconductor.

[0093] Returning again to FIG. 1, the optical feedback loop system 34 is constructed as follows. That is, this optical feedback loop system 34 is made up of an input monitor section comprising an optical branching section 1 a, a gain control means 11 a, an optical isolator 3, an optical coupler 8 and a PD 10; a multiplexing section comprising an optical coupler 8′, an excitation light source 4 and a multiplexer (multiplexing portion) 9; and an amplifying section comprising an EDFA 12, a gain equalizer 13, a fourth wavelength-multiplexed light amplifying basic unit 35, an auxiliary excitation light source 4 a and a connector 5.

[0094] In this arrangement, the optical branching section 1 a is for deriving the output from the rearmost wavelength-multiplexed light amplifying unit 30, and this function is achievable through the use of a 10:1 optical coupler. The gain control means 11 a is for controlling the gains of the plurality of wavelength-multiplexed light amplifying units 30, arranged in a multistage fashion, on the basis of the output information from the wavelength-multiplexed light amplifying unit 30 indicated at (C), and this function is achievable by an optical filter 11, a variable attenuator 11 b and a control section 11 c. This optical filter 11 is for selecting an optical signal with a desired wavelength from the output information, and is constructed as a filter for selecting an optical signal having a wavelength being out of a wavelength band needed for the transmission. Furthermore, the variable attenuator 11 b is for attenuating the optical signal from the optical filter 11 to supply it to the input side of the gain equalizer 13, with this function being obtainable by an optical attenuator. Still furthermore, the control section 11 c is for controlling the wavelength characteristic of the gain, and is for controlling the output of the excitation light source 4.

[0095] With this configuration, from a wavelength-multiplexed optical signal outputted from the optical branching section 1 a, the optical filter 11 selects an optical signal (output information) having an optical frequency which is out of that wavelength-multiplexed band, with the detection of that wavelength being made through a laser resonance. The control section 11 c controls the wavelength characteristic of the gain, and the variable attenuator 11 b attenuates the optical signal and controls the outputs of the excitation light source 4 and the auxiliary excitation light source 4 a. Thus, the output of the excitation light source 4 and the output of the auxiliary excitation light source 4 a to be inputted to the input ports P₁ of the optical branching sections 1 a of the wavelength-multiplexed light amplifying units 30 indicated at (B) to (F) are controlled automatically (APC:Auto Power Control) so that the control of the wavelength characteristics of the gains in all the optical branching sections 1 a to 1 c becomes possible.

[0096] Furthermore, the optical isolator 3 is for conducting the isolation about the optical amplification in the optical feedback loop system 34, and the optical coupler 8 is for performing the optical coupling in which the ratio of the output to the next stage and the input monitor is 10:1, and further the PD 10 is for detecting the input to the fourth wavelength-multiplexed light amplifying basic unit 35, with the input light to this feedback system 34 being coupled in the optical coupler 8 for monitoring.

[0097] Still furthermore, the optical coupler 8′ is for performing the optical coupling with the ratio of an input signal from the previous-stage and a feedback signal being 10:1, and the excitation light source 4 is an LD for excitation, and the multiplexer 9 is for multiplexing excitation light from the excitation light source 4 and a wavelength-multiplexed optical signal together.

[0098] Still furthermore, the EDFA 12 is an amplification fiber, and the gain equalizer 13 is for equalizing an optical signal outputted from the EDFA 12 in optical frequency, with they being interposed between the multiplexer 9 and the signal input port P₂ of the fourth wavelength-multiplexed light amplifying basic unit 35. Accordingly, the irregular amplification gain characteristic of an optical signal is leveled by the gain equalizer 13. Additionally, the auxiliary excitation light source 4 a is a light source for compensation, and supplies light to the input ports P₁ of the optical branching sections 1 a of the wavelength-multiplexed light amplifying units 30 expressed by (B) to (F), so that the control of the gain wavelength characteristics at all the optical branching sections 1 a to 1 c in the wavelength-multiplexed light amplifying units 30 becomes possible.

[0099] Accordingly, the wavelength-multiplexed light amplifying apparatus 25 is made up of an EDFA 12 (an optical amplifying medium) for amplifying input light, wavelength-multiplexed light amplifying units 30 (optical branching/amplifying sections), expressed by (B), (C), (D), (E) and (F), for branching an optical signal outputted from this EDFA 12 into a plurality of optical signals and further for amplifying the respective branched optical signals for compensating for a loss resulting from the branching, and a gain control means 11 a connected to the optical branching/amplifying sections 30 for controlling the gain of the EDFA 12 in accordance with the optical signals from the optical branching/amplifying sections 30.

[0100] Further, the wavelength-multiplexed light amplifying apparatus 25 functions as an optical amplifier and comprises a beam splitting means (beam splitter), an excitation light source (pumping light source) 4, a monitoring means and a control means (gain control means 11 a). The beam splitting means have a portion for amplifying the input light and have a function which split an input light. This function is realized by cooperating the wavelength-multiplexed light amplifying unit 30 indicated by (B), (C), (D), (E) and (F). Moreover, the excitation light source 4 emits a pumping light for pumping optical amplifying portion of the beam splitting means. Additionally, the monitoring means have a function which monitor the output light of the beam splitting means, and this function works by cooperating of the optical isolator 3, the optical coupler 8 and the PD 10. And the more, the gain control means 11 a has a function which controls a power of the excitation light based on the value outputted from the monitoring means.

[0101] The fourth wavelength-multiplexed light amplifying basic unit 35 is for coupling the output of the auxiliary excitation light source 4 a connected through the optical connector 5 and the output of the gain equalizer 13 and for inputting the coupling result to the wavelength-multiplexed light amplifying unit 30 expressed as (B). As well as described above, this unit 35 is composed of optical branching section 1 a and optical amplifying sections 2 a and 2 b. Thus, the wavelength-multiplexed light amplifying unit 30 has the same configuration as that of the first wavelength-multiplexed light amplifying basic unit 31, and the output side of its one optical amplifying section 2 a is connected to the signal input port P₂ of the first wavelength-multiplexed light amplifying basic unit 31 in the forefront unit 30 (indicated at B) of the plurality of wavelength-multiplexed light amplifying units arranged in a multistage fashion.

[0102] Accordingly, the operational flow of this wavelength-multiplexed light amplifying apparatus 25 is as follows. That is, an optical signal outputted from the fourth wavelength-multiplexed light amplifying basic unit 35, indicated at (A) in FIG. 1, is inputted to the wavelength-multiplexed light amplifying unit 30 indicated at (B) where that signal is branched or divided into four. Furthermore, these four divisions of the optical signal are distributed to the wavelength-multiplexed light amplifying unit 30 indicated at (C), the wavelength-multiplexed light amplifying unit indicated at (D), the wavelength-multiplexed light amplifying unit indicated at (E) and the wavelength-multiplexed light amplifying unit indicated at (F), respectively, thereby producing 16 distributions. Likewise, an optical signal outputted from the optical amplifying section 2 a in the fourth wavelength-multiplexed light amplifying basic unit 35 in FIG. 1 undergoes 16 distributions although omitted from the illustration, thus realizing the broadcast function.

[0103] Furthermore, an optical signal outputted from the wavelength-multiplexed light amplifying unit 30 denoted at (C) is inputted to the optical branching section 1 a of the optical feedback loop system 34, and an optical signal from this optical branching section 1 a is inputted to the optical filter 11 to extract an optical signal having a frequency which is out of a frequency area to be wavelength-multiplexed, and the control section 11 c monitors the power of the extracted optical signal and the variable attenuator 11 b adjusts the attenuation, thereby controlling the input of the optical coupler 8′. Additionally, a control signal from this control section 11 c is given to the excitation light source 4 and the auxiliary excitation light source 4 a.

[0104] On the other hand, an optical signal from the other wavelength-multiplexed light amplifying apparatus, coming from the left side in FIG. 1, is inputted through the connector 5 and the optical isolator 3 to the optical coupler 8, and its level is always monitored by the PD 10. Furthermore, in the multiplexer 9, the output of the optical coupler 8′ is multiplexed with an optical signal from the excitation light source 4, and this signal is optically amplified in the EDFA 12, and its optical spectrum is leveled in the gain equalizer 13. An optical signal from the gain equalizer 13 and an optical signal outputted from the auxiliary excitation light source 4 a are coupled to each other in the optical branching section 1 a in the fourth wavelength-multiplexed light amplifying basic unit 35, and again inputted to the optical amplifying section 2 a or the optical amplifying section 2 b.

[0105] As described above, this wavelength-multiplexed light amplifying apparatus 25 internally includes the wavelength-multiplexed light amplifying units 30 connected in a multistage fashion and the optical feedback loop system 34, and, for the purpose of the constant-gain control, the reduction of noise and the leveling of gain band, monitors and controls the input power and the output power through the use of the excitation light source 4, the multiplexer 9, the EDFA 12, the gain equalizer 13 and others. Additionally, the light compensation becomes possible through the use of the optical branching section 1 a (1:1 optical coupler) of the fourth wavelength-multiplexed light amplifying basic unit 35 in the optical feedback loop system 34. Still additionally, this wavelength-multiplexed light amplifying apparatus 25 feedbacks an optical signal from one port to constitute a laser resonator (the optical filter 11, the control section 11 a, the variable attenuator 11 b), and handles the wavelength characteristic of the gain by adjusting the attenuation of the variable attenuator 11 b in the optical feedback loop system 34.

[0106] Moreover, this wavelength-multiplexed light amplifying apparatus 25 can also be expressed as follows looking at the function of the optical add-and-drop apparatus shown in FIG. 2. For instance, in FIG. 2, it is applicable as M=64, N=10 and L=32. That is, this wavelength-multiplexed light amplifying apparatus 20 is a wavelength-multiplexed light amplifying apparatus to be connected to a branching port of a multiplexing/branching unit which receives a wavelength-multiplexed optical signal comprising optical signals of 64 kinds of wavelengths and a wavelength-multiplexed optical signal comprising optical signals of 10 kinds of wavelengths and branches the input into a wavelength-multiplexed optical signal comprising optical signals of (64+10−32) kinds of wavelengths and a wavelength-multiplexed optical signal comprising optical signals of 32 kinds of wavelengths to output them, and this wavelength-multiplexed light amplifying apparatus 20 has a plurality of wavelength-multiplexed light amplifying basic units 30 b (basic units which will be described later with reference to FIG. 7) each comprising an optical branching section 1 a including two input ports P₁ and P₂ and two output ports P₃ and P₄ for receiving a wavelength-multiplexed optical signal comprising optical signals of 32 kinds of wavelengths through the signal input port P₂ forming one input port of the two input ports to output wavelength-multiplexed optical signals through the two output ports P₃ and P₄ and optical amplifying sections 2 a and 2 b respectively connected to the two output ports P₃ and P₄ of the optical branching section 1 a. These wavelength-multiplexed light amplifying basic units 30 b are connected in a multistage fashion in a manner that the signal input port P₂ of the optical branching section 1 a is connected to the output side of the optical amplifying section 2 c in the preceding (previous-stage) wavelength-multiplexed light amplifying basic unit 30 b, and the excitation light source 4 is connected to the other input port P₁ of the optical branching section 1 a in the forefront unit of the plurality of wavelength-multiplexed light amplifying basic units 30 b. Additionally, this easily realizes a broadcast function, and permits application for an optical add-and-drop apparatus.

[0107] Concretely, if using only the branch into two, the user purchases three parts, that is, (A), (B) and (C) in FIG. 1. Furthermore, if the branch into five is needed later, the user further increases (D). Similarly, up to the branch into 16 is possible if the user adds (D), (E) and (F). For the branch into 17, (B) and (C) are connected to the empty port P₃ of (A) of the fourth wavelength-multiplexed light amplifying basic unit 35 in the optical feedback loop system 34. In this way, a maximum of branch into 32 becomes possible, and the hardware is expandable according to the demands. Furthermore, if the hardware operating currently is left as it is and the user additionally purchases the hardware when he wants the extension of the service, it is possible to meet the communication requests.

[0108] With this configuration, an optical signal from the fourth wavelength-multiplexed light amplifying basic unit 35 is broadcasted and distributed into a desired number, the information on the optical signal outputted from the rearmost unit is fed back to accomplish the wavelength-multiplexed light output control appropriately.

[0109] Concretely, an optical signal of 18 dBm inputted to the optical branching section 1 a of the fourth wavelength-multiplexed light amplifying basic unit 35 becomes 15 dBm at the output of the optical branching section 1 a, and gets a gain of 3 dB at the optical amplifying section 2 a so that a power of 18 dBm is outputted from the optical amplifying section 2 a. This optical signal of 18 dBm is branched into two in the optical branching section 1 a of the first wavelength-multiplexed light amplifying basic unit 31 in the wavelength-multiplexed light amplifying unit 30 indicated at (B), and becomes 15 dBm at the output of the optical branching section 1 a, while it again takes a gain of 3 dB at the optical amplifying section 2 a to regain 18 dBm at the output of the optical amplifying section 2 a. Additionally, this optical signal of 18 dBm is also branched into two in the optical branching section 1 b of the second wavelength-multiplexed light amplifying basic unit 32 in the wavelength-multiplexed light amplifying unit 30 indicated at (B), and takes a gain of 3 dB to reach 18 dBm before outputted. This optical signal is inputted to the wavelength-multiplexed light amplifying unit 30 indicated at (C), and similarly branched and amplified before inputted into the optical feedback loop system 34.

[0110] Thus, the information (output information) on the outputted optical signal from the rearmost unit is fed back and, in the gain control means 11 a, the output information works for the input side optical coupler 8′, the excitation light source 4 and the auxiliary excitation light source 4 a, which ensures appropriate control.

[0111] In this way, without employing a configuration in which another optical distribution system is prepared to distribute an optical signal amplified, this wavelength-multiplexed light amplifying apparatus 25 is arranged such that amplifying mediums are dispersed in the optical distribution system as taken in the wavelength-multiplexed light amplifying units 30 indicated at (B) to (F); whereupon the optical signal is gradually distributed after amplified, thereby dispersing the loss. Accordingly, as compared with a method in which an optical signal is amplified at a time and distributed, the increase in optical power does not occur locally, thereby presenting a higher-efficiency characteristic. That is, in this wavelength-multiplexed light amplifying apparatus 25, the signal optical power assumes approximately 18 dBm (=63 mW) at most, and decreases significantly as compared with the maximum optical power occurring when a wavelength-multiplexed optical signal is amplified at a time. Additionally, unlike the case in which it is amplified at a time, a large loss of the signal optical power does not occur. In this case, the excess loss does not constitute a main factor to the loss.

[0112] Furthermore, as described above, in this wavelength-multiplexed light amplifying apparatus 25, since the amplifying mediums (optical amplifying sections 2 a to 2 f) are disconnected effectively due to the effective loss due to the distribution, stable operation is realizable without relying on the optical isolator 3. Additionally, in this wavelength-multiplexed light amplifying apparatus 25, in the wavelength-multiplexed light amplifying units 30 connected as a distribution system in a multistage fashion, since the optical branching sections 1 a to 1 c (1:1 optical couplers) are connected utilizing a free portion of the two input ports, it is possible to eliminate the need for the multiplexer 9 which multiplexes the excitation light and the signal light, or to decrease the required number. Still additionally, since the light from the excitation light source 4 is equally distributed utilizing the optical branching sections 1 a to 1 c (1:1 optical couplers), it is possible to supply the equal excitation optical power to the respective optical amplifying sections 2 a to 2 f with a simple configuration. On the other hand, if the optical isolator 3 is used on the input side of the component, in addition to avoiding the unstable operation resulting from the reflection returning light, it is possible to shut off the reverse-direction ASE (Amplified Spontaneous Emission) coming from the output side to the input side, thereby preventing the saturation of the optical amplifier due to the reverse-direction ASE.

[0113] Moreover, this wavelength-multiplexed light amplifying apparatus 25 can offer easy expansion in the in-service. That is, if the user additionally purchases this wavelength-multiplexed light amplifying unit 30 after the introduction of this wavelength-multiplexed light amplifying apparatus 25, the expansion becomes easily feasible in the number of branches, and becomes possible without interrupting the current service, thus presenting higher-quality service. Additionally, in the broadcast system 20 shown in FIG. 2, a wavelength-multiplexed optical signal is output through each port, and if only a desired wavelength is selected therefrom through a wavelength selecting filter, the optical signal distribution function is realizable, thus providing the broadcast function.

[0114] Furthermore, in this wavelength-multiplexed light amplifying apparatus 25, the optical part in each component is composed of the optical amplifying sections (EDFA) 2 a to 2 f, the optical couplers 1 a to 1 c, the optical switch, the excitation light source 4 and the optical isolator 3, which have the equal characteristic; therefore, this system 25 is suitable for the mass production and excellent expansion is attainable. For instance, in FIG. 1, the units (B) to (F) other than (A), have the completely same configuration, thus enabling the mass production.

[0115] Still furthermore, in the wavelength-multiplexed light amplifying units 30, since the light from the excitation light source 4 in the forefront unit is distributed equally, if the excitation light source 4 in the forefront unit has a power sufficient to excite the optical amplifying sections 2 a to 2 f in the rearmost wavelength-multiplexed light amplifying unit 30, there is no need to provide the excitation light source 4 in the intermediate wavelength-multiplexed light amplifying units 30. For instance, if an optical signal outputted from (A) has a power sufficient to excite the following amplifying units, the excitation light source 4 is not required in (B) to (F), while the excitation light source 4 can be placed in (B) when needed. Additionally, the excitation light source 4 can also be connected to all the amplifying units of (A) to (C). Accordingly, it is possible to employ the exciting method flexibly. Still additionally, it is also appropriate that one large-output excitation light source is prepared and is connected to an excitation portion (for example, the port P₁ of the optical branching section 1 a) of each of the wavelength-multiplexed light amplifying units 30. This is equivalent to the case in which, in a gas station, gasoline is distributed from a gasoline tank to many motor vehicles at a time.

[0116] Moreover, the wavelength-multiplexed light amplifying apparatus 25 is made to receive laser light (compensatory light) from the auxiliary excitation light source 4 a under the control of one portion P₁ of the forefront unit (the fourth wavelength-multiplexed light amplifying basic unit 35 indicated at (A) in FIG. 1), and the laser light by the distribution system is outputted equally to all the ports and the control of the wavelength characteristic of the gain at all the branching ports becomes possible. Likewise, since the magnitude of the excitation optical power is equal in the respective distribution systems, in a manner that only one port is represented and monitored so that the feedback control is implemented for the excitation light source 4 in the forefront unit to control the wavelength characteristic of the gain, the wavelength characteristics of the gain at all the ports are controllable. Additionally, since the gains at all the ports vary equally, the port to be fed back is only one, and if the excitation light is altered to maintain the gain at each of the ports constant, it is possible to maintain the gains at all the ports constant with a simple configuration.

[0117] In addition, it is also possible to change the configuration of the wavelength-multiplexed light amplifying unit 30.

[0118]FIG. 5 is a block diagram showing a configuration of another wavelength-multiplexed light amplifying unit according to the first embodiment of this invention. This wavelength-multiplexed light amplifying unit, designated generally at numeral 30 a in FIG. 5, comprises three basic units, that is, a first wavelength-multiplexed light amplifying basic unit 31, a second wavelength-multiplexed light amplifying basic unit 32 and a third wavelength-multiplexed light amplifying basic unit 33, and further comprises an optical isolator 3 and an excitation light source 4 on the preceding side of the first wavelength-multiplexed light amplifying basic unit 31, and still further comprises optical isolators 3 coupled to two ports P₃ and P₄ on the output side of the second wavelength-multiplexed light amplifying basic unit 32 and optical isolators 3 coupled to two ports P₃ and P₄ on the output side of the third wavelength-multiplexed light amplifying basic unit 33. Additionally, each of an input port P₁ of an optical branching section 1 b and an input port P₂ of an optical branching section 1 c is connectable through a connector 5 to an auxiliary excitation light source 4 a. Still additionally, the input side optical isolator 3 is connected through a connector 5 to the preceding wavelength-multiplexed light amplifying unit 30 a, while the four output side optical isolators 3 are connected through connectors 5 to the following wavelength-multiplexed light amplifying unit 30 a. These parts are the same as those described above, and the same description will be omitted for brevity. Furthermore, since the optical isolators 3 are inserted into these four places, the resistance to the reflection improves. This configuration operates as well as the above-described wavelength-multiplexed light amplifying unit 30 and offers the same effects.

[0119] Furthermore, looking at the point that, in this wavelength-multiplexed light amplifying unit 30 a, an optical branching section 1 a and an optical amplifying section 2 a are connected to each other, the following unit combination is considered as an amplifying basic unit. FIG. 6 is a block diagram showing another wavelength-multiplexed light amplifying unit according to the first embodiment of this invention. In a wavelength-multiplexed light amplifying apparatus 25, looking at the point that the optical branching section 1 a and the optical amplifying sections 2 a and 2 b are connected to each other, this wavelength-multiplexed light amplifying basic unit, designated generally at numeral 30 b in FIG. 6, can be considered to constitute a multistage wavelength-multiplexed light amplifying unit, and this is put to use. That is, the wavelength-multiplexed light amplifying basic unit 30 b shown in FIG. 6 is provided with the optical branching section 1 a and the optical amplifying sections 2 a and 2 b, and further equipped with an optical isolator 3 and an excitation light source 4 on the input port P₂ side of the optical branching section 1 a. In this case, the optical isolator 3 can be omitted for convenience in design. Additionally, the input side of the optical isolator 3 and the output sides of the optical amplifying sections 2 a and 2 b are connected through connectors 5 to the preceding and following wavelength-multiplexed light amplifying basic units 30 b, respectively. Still additionally, the optical branching section 1 a in FIG. 6 is constructed as a 1:1 optical coupler, while the optical amplifying sections 2 a and 2 b are constructed as an optical amplifying section having a gain equivalent to the branch loss in the optical branching loss 1 a or having a gain lower than that branch loss, besides, in this wavelength-multiplexed light amplifying basic unit 30 b, the optical branching section 1 a, the optical amplifying sections 2 a and 2 b and a connecting section between the optical branching section 1 a and the optical amplifying sections 2 a and 2 b can also be constructed using an optical waveguide made of a glass or a semiconductor. The other parts are the same as those described above, and the same description thereof will be omitted for simplicity.

[0120] Thus, the wavelength-multiplexed light amplifying unit comprises a plurality of wavelength-multiplexed light amplifying basic units 30 b, each of which is composed of an optical branching section 1 a having two input ports P₁ and P₂ and two output ports P₃ and P₄ for receiving a wavelength-multiplexed optical signal comprising optical signals of a plurality of wavelengths through the signal input port P₂ forming one of the two input ports P₁ and P₂ to output the wavelength-multiplexed optical signal through the two output ports P₃ and P₄ and optical amplifying sections 2 a and 2 b respectively connected to the two output ports P₃ and P₄ of the optical branching section 1 a, with these wavelength-multiplexed light amplifying basic units 30 b being connected in a multistage fashion in a manner that the signal input port P₂ of the optical branching section 1 a of one wavelength-multiplexed light amplifying basic unit 30 b is connected to the output side of the optical amplifying section 2 a, 2 b in the preceding wavelength-multiplexed light amplifying basic unit 30 b, an excitation light source 4, a multiplexer 9 for multiplexing the excitation light from the excitation light source 4 and the wavelength-multiplexed optical signal, a gain equalizer 13 interposed between the multiplexer 9 and the signal input port P₂ of the forefront unit 30 b of the plurality of wavelength-multiplexed light amplifying basic units 30 b disposed in a multistage fashion, and a gain control means 11 a for implementing gain control of the plurality of wavelength-multiplexed light amplifying basic units 30 b, disposed in a multistage fashion, on the basis of output information from the rearmost unit 30 b of the plurality of wavelength-multiplexed light amplifying basic units 30 b.

[0121] Accordingly, instead of the wavelength-multiplexed light amplifying unit 30 comprising the first wavelength-multiplexed light amplifying basic unit 31 to the third wavelength-multiplexed light amplifying basic unit 33, it is also considered that the multistage connection of these wavelength-multiplexed light amplifying basic units 30 b produces a configuration of a wavelength-multiplexed light amplifying apparatus.

[0122] In addition, in the case connected in a multistage fashion, as well as the system described with reference to FIG. 1, it is also possible that the auxiliary excitation light source 4 a is connected to the other input port P₁ of at least one of the plurality of wavelength-multiplexed light amplifying basic units 30 b, or that the optical isolator 3 is connected to the signal input port P₂ therein. Furthermore, in this case, the gain control means 11 a can also be composed of an optical filter 11 and an optical attenuator (variable attenuator 11 b) for attenuating an optical signal with a desired wavelength, extracted in the optical filter 11, for laser resonance to supply the attenuated optical signal to the input side of the gain equalizer 13. This optical filter 11 is constructed as a filter made to select an optical signal with a wavelength being out of a given wavelength band needed for transmission.

[0123] Incidentally, only one of these wavelength-multiplexed light amplifying basic units can constitute a wavelength-multiplexed light amplifying apparatus.

[0124]FIG. 7 is an illustration of a further wavelength-multiplexed light amplifying apparatus according to the first embodiment of this invention. The wavelength-multiplexed light amplifying apparatus, designated generally at numeral 31 a in FIG. 7, is made up of a wavelength-multiplexed light amplifying basic unit 30 b and an optical feedback loop system 34. In this configuration, let it be assumed that, as indicated by an broken line in FIG. 7, the output light from a fourth wavelength-multiplexed light amplifying basic unit 35 in the optical feedback loop system 34 is inputted directly to an optical branching section 1 a in the optical feedback loop system 34.

[0125] The fourth wavelength-multiplexed light amplifying basic unit 35 in the optical feedback loop system 34 shown in FIG. 7 is equipped with the optical branching section 1 a and optical amplifying sections 2 a and 2 b. The optical branching section 1 a is a 1:1 optical coupler, while the optical amplifying sections 2 a and 2 b are an optical amplifying section having a gain equivalent to the branch loss in the optical branching section 1 a or having a gain lower than that branch loss. Additionally, the optical branching section 1 a, the optical amplifying sections 2 a and 2 b and a connecting section between the optical branching section 1 a and the optical amplifying sections 2 a and 2 b can also be constructed using an optical waveguide made of a glass or a semiconductor. Still additionally, an optical isolator 3 is coupled through optical couplers 8 and 8′, a multiplexer 9 and an EDFA 12 to the signal input port P₂ in the fourth wavelength-multiplexed light amplifying basic unit 35, while an auxiliary excitation light source 4 a is coupled to the other input port P₁ therein.

[0126] On the other hand, the optical feedback loop system 34 is made up of an optical branching section 1 a, a gain control means 11 a, an optical isolator 3, an optical coupler 8, an LD 10, an optical coupler 8′, an excitation light source 4, a multiplexer 9, an EDFA 12, a gain equalizer 13, an auxiliary excitation light source 4 a, a connector 5, and the fourth wavelength-multiplexed light amplifying basic unit 35. The gain control means 11 a is composed of an optical filter 11 for selecting an optical signal with a desired wavelength from the output information, an optical attenuator (variable attenuator 11 b) for attenuating the optical signal with the desired wavelength, extracted through the optical filter 11, for the laser resonance to supply the attenuated optical signal to the input side of the gain equalizer 13, and a control section 11 c , with this optical filter 11 being made to select an optical signal with a wavelength being out of a given wavelength band needed for transmission. The other parts are the same as those described above, and the same description thereof will be omitted for brevity.

[0127] Thus, this wavelength-multiplexed light amplifying apparatus 31 a comprises a fourth wavelength-multiplexed light amplifying basic unit 35 composed of an optical branching section 1 a having two input ports P₁ and P₂ and two output ports P₃ and P₄ for receiving a wavelength-multiplexed optical signal comprising optical signals of a plurality of wavelengths through a signal input port P₂ forming one of the two input ports P₁ and P₂ to output the wavelength-multiplexed optical signal through the two output ports P₃ and P₄ and optical amplifying sections 2 a and 2 b respectively connected to the two output ports P₃ and P₄ of the optical branching section 1 a, an excitation light source 4, a multiplexer 9 for multiplexing the excitation light from the excitation light source 4 and the wavelength-multiplexed optical signal, a gain equalizer 13 interposed between the multiplexer 9 and the signal input port P₂ of the fourth wavelength-multiplexed light amplifying basic unit 35, and a gain control means 11 a for implementing gain control of the fourth wavelength-multiplexed light amplifying basic unit 35 on the basis of the output information from the fourth wavelength-multiplexed light amplifying basic unit 35.

[0128] Furthermore, with this configuration, in FIG. 7, an optical signal from the fourth wavelength-multiplexed light amplifying basic unit 35 is inputted directly to the optical branching section 1 a in the optical feedback loop system 34 as indicated by a broken line, thereby accomplishing appropriate control, while, for the user's expansion, another amplifying unit can be connected through a connector 5 to the optical amplifying section 2 b of the fourth wavelength-multiplexed light amplifying basic unit 35.

[0129] Still furthermore, apart from that indicated by the broken line, in FIG. 7, in the case in which an optical signal from the fourth wavelength-multiplexed light amplifying basic unit 35 is inputted to the wavelength-multiplexed light amplifying basic unit 30 b, needless to say, the optical signal is distributed into two in the wavelength-multiplexed light amplifying basic unit 30 b indicated at (B), and the output optical signal information from this wavelength-multiplexed light amplifying basic unit 30 b is fed back, thereby accomplishing appropriate wavelength-multiplexed light output control. Additionally, the expansion of branch becomes possible through connectors 5.

[0130] In this way, with only the fourth wavelength-multiplexed light amplifying basic unit 35, an optical feedback loop system is constructible, so that it is possible to offer it as simplified equipment. Additionally, if the user purchases the wavelength-multiplexed light amplifying basic unit 30 b after the introduction of the wavelength-multiplexed light amplifying apparatus 31 a, the increase in the number of branches becomes easily feasible without interrupting the current service. (A1) Description of a First Modification of the First Embodiment of the Invention

[0131]FIG. 8 is an illustration of a wavelength-multiplexed light amplifying apparatus according to a first modification of the first embodiment of this invention. In FIG. 8, a wavelength-multiplexed light amplifying apparatus, is designated generally at numeral 31 b, is for distributing one optical signal in 32 directions (distribution into 32 is illustrated), and employs control by an optical feedback. Additionally, as well as the first embodiment, the wavelength-multiplexed light amplifying apparatus 31 b principally uses an optical add-and-drop function at a place required.

[0132] In FIG. 8, the wavelength-multiplexed light amplifying apparatus 31 b is made up of a plurality of wavelength-multiplexed light amplifying units 30 c (indicated at (A) to (E) in FIG. 8)] connected in a multistage fashion, and an optical feedback loop system 34 a (a left-side part in FIG. 8). Furthermore, each of the wavelength-multiplexed light amplifying units 30 c is composed of a first wavelength-multiplexed light amplifying basic unit 31, a second wavelength-multiplexed light amplifying basic unit 32 and a third wavelength-multiplexed light amplifying basic unit 33.

[0133] A difference of this wavelength-multiplexed light amplifying unit 30 c from the above-described wavelength-multiplexed light amplifying unit 30 is that a connector this wavelength-multiplexed light amplifying unit 30 c uses differs and an optical feedback loop system 34 a uses an oblique connectors 6 a and 6 b, different in configuration from that described above, between an auxiliary excitation light source 4 a and a fourth wavelength-multiplexed light amplifying basic unit 35 as shown in FIG. 8. The oblique connectors 6 a and 6 b can prevent useless light amplification by inhibiting the occurrence of light reflection on their connector surfaces. Furthermore, in at least two ((A) and (B) in FIG. 18)] of the aforesaid plurality of wavelength-multiplexed light amplifying units 30 c, the output sides of optical amplifying sections 2 c and 2 d of the second and third wavelength-multiplexed light amplifying basic units 32 and 33 in the preceding side wavelength-multiplexed light amplifying unit 30 c (A)] and the input sides of signal input ports P₂ of the first wavelength-multiplexed light amplifying basic unit 31 in the following side wavelength-multiplexed light amplifying units 30 c ((B) to (E))] are connected through oblique connectors 6 a and 6 b to each other. Likewise, in FIG. 8, an optical signal outputted from the optical amplifying section 2 b in the fourth wavelength-multiplexed light amplifying basic unit 35 is, although omitted, distributed through optical connectors 6 b into 16. As FIG. 8 shows, these wavelength-multiplexed light amplifying units 30 c are connected in a multistage fashion in a manner that the signal input port P₁ of the optical branching section 1 a of the first wavelength-multiplexed light amplifying basic unit 31 in one wavelength-multiplexed light amplifying unit 30 c is connected to the output side of the optical amplifying section 2 c (2 d, 2 e, 2 f) of the second wavelength-multiplexed light amplifying basic unit 32 or the third wavelength-multiplexed light amplifying basic unit 33 in the wavelength-multiplexed light amplifying unit 30 c preceding the one wavelength-multiplexed light amplifying unit 30 c. The other components marked with the same reference numerals as those used above are identical to or correspond in function to those described above, and the description will be omitted for simplicity.

[0134]FIG. 9 is a block diagram showing a configuration of a wavelength-multiplexed light amplifying unit according to the first modification of the first embodiment of this invention. In FIG. 9, the wavelength-multiplexed light amplifying unit 30 c is made up of the three basic units, that is, the first wavelength-multiplexed light amplifying basic unit 31 comprising an optical branching section 1 a and optical amplifying sections 2 a and 2 b, the second wavelength-multiplexed light amplifying basic unit 32 comprising an optical branching section 1 b and optical amplifying sections 2 c and 2 d and the third wavelength-multiplexed light amplifying basic unit 33 comprising an optical branching section 1 c and optical amplifying sections 2 e and 2 f, and further includes an optical isolator 3 and an excitation light source 4 preceding the first wavelength-multiplexed light amplifying basic unit 31. Additionally, connectors 5 are provided at the input port P₁ of the optical branching section 1 b and at the input port P₂ of the optical branching section 1 c, respectively, while a connector 6 a is placed on the input side of the optical isolator 3 and connectors 6 b are placed on the output sides of the optical amplifying sections 2 c, 2 d, 2 e and 2 f.

[0135] This configuration operates similarly to the above-described operation, and additionally prevents useless light amplification because the connection made through the oblique connectors 6 a and 6 b prevents the light reflection on the connector surfaces.

[0136] Incidentally, it is also possible to, instead of the plurality of wavelength-multiplexed light amplifying units 30 c shown in FIG. 8, employ wavelength-multiplexed light amplifying basic units 30 b (see FIG. 6) comprising a set of units. That is, in two wavelength-multiplexed light amplifying basic units 30 b, the connections between the output sides (see FIG. 6) of the optical amplifying sections 2 a and 2 b in the preceding side wavelength-multiplexed light amplifying basic unit 30 b and the input sides of the signal input ports P₂ in the following side wavelength-multiplexed light amplifying basic units 30 c can also be made through the oblique connectors 6 a and 6 b.

[0137] Furthermore, it is also possible that a leakage optical signal is again inputted as an excitation light source in the other wavelength-multiplexed light amplifying basic unit 30 c.

[0138]FIG. 16 is an illustration of a different wavelength-multiplexed light amplifying apparatus according to the first modification of the first embodiment of this invention, where the wavelength-multiplexed light amplifying apparatus 31 c has a configuration in which an excitation light re-inputting section 66 is added to the wavelength-multiplexed light amplifying apparatus 31 b shown in FIG. 8 in a state where oblique connectors 6 b and 6 a are interposed therebetween.

[0139] This excitation light re-inputting section 66 is interposed between a fourth wavelength-multiplexed light amplifying basic unit 35 and a wavelength-multiplexed light amplifying unit 30 c, designated at (A), placed at the forefront position (which will be referred to hereinafter as a wavelength-multiplexed light amplifying 30 c(A)) for removing a wavelength-multiplexed optical signal component from a leakage optical signal outputted from the fourth wavelength-multiplexed light amplifying basic unit 35 to extract excitation light so that this excitation light is branched into a plurality of lights. The excitation light re-inputting section 66 is made up of a wavelength-multiplexed optical signal removing filter 11 d and an optical branching (1:1 CPL) 1 a.

[0140] The wavelength-multiplexed optical signal removing filter 11 d is for eliminating the wavelength-multiplexed optical signal component from the leakage optical signal outputted from the fourth wavelength-multiplexed light amplifying basic unit 35 to extract excitation light.

[0141] Furthermore, the optical branching section 1 a is connected to the wavelength-multiplexed optical signal removing filter 11 d, and its output side is further connected to a signal input port of the second wavelength-multiplexed light amplifying basic unit 32 in the wavelength-multiplexed light amplifying unit 30 c(A) and to a signal input port of the third wavelength-multiplexed light amplifying basic unit 33, thus putting the excitation light extracted by the wavelength-multiplexed optical signal removing filter 11 d into the wavelength-multiplexed light amplifying unit 30 c(A). For example, this function is realizable with a 1:1 optical coupler.

[0142] With this configuration, after passing through the connectors 6 b and 6 a, of large leakage excitation light developing from the fourth wavelength-multiplexed light amplifying basic unit 35, the signal light is blocked by the wavelength-multiplexed optical signal removing filter 11 d, and the optical signal subjected to the filtering is branched into two which in turn, are inputted through the oblique connectors 6 b and 6 a to an input port P₁ of an optical branching section 1 b and an input port P2 of an optical branching section 1 c in the wavelength-multiplexed light amplifying unit 30 c (A) (see FIG. 3).

[0143] Accordingly, since the leakage excitation light can be used again so that efficient amplification becomes also possible. Additionally, since the leakage excitation light is inputted to the most upstream side of the wavelength-multiplexed light amplifying units 30 c connected in a multistage fashion, useless light amplification is preventable.

[0144] Furthermore, in addition to the wavelength-multiplexed light amplifying apparatus 31 b (see FIG. 8), the excitation light re-inputting section 66 can also be connected to the wavelength-multiplexed light amplifying apparatus 31 a shown in FIG. 7. In this case, the wavelength-multiplexed optical signal removing filter 11 d removes a wavelength-multiplexed optical signal component from a leakage optical signal outputted from the fourth wavelength-multiplexed light amplifying basic unit 35 to extract excitation light, while the optical branching section 1 c is connected to the wavelength-multiplexed optical signal removing filter 11 d, and its output side is connected to the other input port of the wavelength-multiplexed light amplifying unit 35, with the excitation light extracted by the wavelength-multiplexed optical signal removing filter 11 d is inputted to the wavelength-multiplexed light amplifying basic unit 30 b.

[0145] Additionally, this excitation light re-inputting section 66 can also be used for the wavelength-multiplexed light amplifying apparatus shown in FIGS. 1, 10, 11 and 13.

[0146] Thus, since the leakage excitation light can be reused, similarly efficient amplification is achievable. (A2) Description of a Second Modification of the First Embodiment of the Invention

[0147]FIG. 10 is an illustration of a wavelength-multiplexed light amplifying apparatus according to a second modification of the first embodiment of this invention. In FIG. 10, a wavelength-multiplexed light amplifying apparatus, designated generally at numeral 31 c, is for distributing one optical signal in 32 directions (of these, 16 distributions illustrated), and employs optical feedback control, principally uses an optical add-and-drop function at a place required.

[0148] In FIG. 10, the wavelength-multiplexed light amplifying apparatus 31 c is made up of a plurality of wavelength-multiplexed light amplifying units 30 d arranged in a multistage fashion and an optical feedback loop system 34 b. The difference of this wavelength-multiplexed light amplifying unit 30 d from the above-described wavelength-multiplexed light amplifying unit 30 is that all the free ports of this wavelength-multiplexed light amplifying unit 30 d are terminated. Additionally, the optical feedback loop system 34 b is provided with an optical branching section 1 a, a gain control means 11 a, an optical isolator 3, an optical coupler 8, an LD 10, an optical coupler 8′, an excitation light source 4, a multiplexer 9, an EDFA 12 and a gain equalizer 13, and further equipped with a fourth wavelength-multiplexed light amplifying basic unit 35 b. A port P₁ of this fourth wavelength-multiplexed light amplifying basic unit 35 b is terminated by a terminator 14, and this optical feedback loop system 34 b does not have an auxiliary excitation light source 4 a (see FIG. 1 and other illustrations), but the excitation light source 4 has a power sufficient to excite the rearmost wavelength-multiplexed light amplifying unit 30 d. The other components marked with the same reference numerals as those used above are identical to or correspond in function to those described above, and the description will be omitted for simplicity.

[0149] Furthermore, these wavelength-multiplexed light amplifying units 30 d are arranged in a multistage fashion in a manner that the signal input port P₁ of the optical branching section 1 a of the first wavelength-multiplexed light amplifying basic unit 31 in one wavelength-multiplexed light amplifying unit 30 d is connected to the output side of the optical amplifying section 2 c (2 d, 2 e, 2 f) of the second wavelength-multiplexed light amplifying basic unit 32 or the third wavelength-multiplexed light amplifying basic unit 33 in the wavelength-multiplexed light amplifying unit 30 d preceding the amplifying unit 30 d. Similarly, an optical signal outputted from the optical amplifying section 2 b in the fourth wavelength-multiplexed light amplifying basic unit 35 b shown in FIG. 10 is, although omitted from the illustration, distributed into 16, thereby realizing the broadcast function.

[0150] This configuration operates similarly to that described above to offer the same effects. Additionally, since the optical power from the excitation light source 4 is distributed equally, there is no need to install the excitation light source 4 in the intermediate wavelength-multiplexed light amplifying units 30 d.

[0151] (B) Description of a Second Embodiment of the Invention

[0152] It is also possible to employ a feedback loop system control method different from that in the first embodiment. That is, it is also possible to control the feedback loop system in an electrical area without controlling it in an optical area.

[0153]FIG. 11 is an illustration of a wavelength-multiplexed light amplifying apparatus according to a second embodiment of this invention. A wavelength-multiplexed light amplifying apparatus, designated generally at reference numeral 41 in FIG. 11, is for distributing one optical signal in 32 directions (of these, 16 distributions illustrated), and uses control based upon an optical feedback. Furthermore, the wavelength-multiplexed light amplifying apparatus 41 in FIG. 11 is mainly used as an optical add-and-drop apparatus.

[0154] In FIG. 11, the wavelength-multiplexed light amplifying apparatus 41 is made up of a plurality of wavelength-multiplexed light amplifying units 30 ((B) to (F))] connected in a multistage fashion and an optical feedback loop system 42 (a left-side part in FIG. 11). Additionally, these wavelength-multiplexed light amplifying units 30 are arranged in a multistage fashion in a manner that the signal input port P₁ of the optical branching section 1 a of a first wavelength-multiplexed light amplifying basic unit 31 in one wavelength-multiplexed light amplifying unit 30 is connected to the output side of an optical amplifying section 2 c (2 d, 2 e, 2 f) of a second wavelength-multiplexed light amplifying basic unit 32 or a third wavelength-multiplexed light amplifying basic unit 33 in the preceding wavelength-multiplexed light amplifying unit 30. Accordingly, when viewed from each of connectors 5 on the output sides, the branch and the amplification are conducted alternately. The other components marked with the same reference numerals as those used above are identical to or correspond in function to those described above, and the description thereof will be omitted for brevity.

[0155] Meanwhile, the optical feedback loop system 42 in FIG. 11 is constructed as follows. That is, this optical feedback loop system 42 is made up of an output monitor system section including an optical branching section 1 a and an optical filter 100 a, a PD 10 a, a gain control means 15, an input monitor system section including an optical isolator 3, an optical coupler 8 and an optical filter 100 b, a PD 10, and an optical signal outputting section including an excitation light source 4, a multiplexer 9, an EDFA 12, a gain equalizer 13, a fourth wavelength-multiplexed light amplifying basic unit 35, an auxiliary excitation light source 4 a and a connector 5.

[0156] In this configuration, the optical branching section 1 a, the PD 10 a, the optical isolator 3, the optical coupler 8, the PD 10, the excitation light source 4, the multiplexer 9, the EDFA 12, the gain equalizer 13, the fourth wavelength-multiplexed light amplifying basic unit 35, the auxiliary excitation light source 4 a and the connector 5 are identical to or correspond in function to those described above, respectively, and the description thereof will be omitted for brevity. Furthermore, the PD 10 a has a function to output-monitor an optical signal, inputted to this optical feedback loop system 42, through the optical branching section 1 a and the optical filter 100 a.

[0157] On the other hand, to monitor an input level, the optical filter 100 b is provided between the PD 10 and the optical coupler 8.

[0158] This optical filter 100 b allows a passing of a signal light component as well as removes the excitation light component from the optical signal outputted from the optical coupler 8. And this function is also realized by using a long wavelength pass filter or a band pass filter, and the long wavelength pass filter is more profitable than the band pass filter as the optical filter 100 b.

[0159] Thus, the optical amplifier of the present invention comprises an optical filter 100 a, which is provided on the input side of above-described monitor (PD 10 a), allows a passing of a signal light component as well as removes the excitation light component from the optical signal.

[0160] Further, the optical amplifier of the present invention comprises the optical filter 100 b, which is provided between the optical coupler 8 and the PD 10, allows a passing of a signal light component as well as removes the excitation light component from the input light.

[0161] In other words, the wavelength-multiplexed light amplifying apparatus of the present invention comprises an optical filter 100 a, which is provided on the input side of above-described monitor (PD 10 a), allows a passing of a signal light component as well as removes the excitation light component from the optical signal outputted from the rearmost wavelength-multiplexed light amplifying unit 30 (indicated at (C))] of the plurality of wavelength-multiplexed light amplifying basic units 30 (indicated at (B)-(F))] arranged in a multistage fashion.

[0162] Further, the wavelength-multiplexed light amplifying apparatus of the present invention comprises an input monitor means and the input monitor means includes an optical filter 100 b which allows a passing of a signal light component from the input light as well as removes the excitation light component from the input light.

[0163] From the foregoing, as for an output monitoring, one of the output lights from the optical branching section 1 a in the optical feedback loop system 42 is inputted to the optical filter 100 a and the component of the excitation light.

[0164] And as for the input monitoring, in the optical coupler 8, the light outputted from the optical isolator 3 is split into two. And in the optical filter 100 b, the component of the wavelength band of the excitation light of one of the split light is inputted to the gain control means 15.

[0165] In this manner, by eliminating the excitation light component, an accuracy of the output monitoring and the input monitoring increases.

[0166] The gain control means 15 produces a gain control signal on the basis of the aforesaid output information and the detection result by the PD 10, and supplies the produced gain control signal to the excitation light source 4 provided on the input side of the gain equalizer 13 for controlling the excitation condition of the excitation light source 4. The gain control means 15 includes a control section 15 a. The control section 15 a receives an electric signal to control the excitation condition of the excitation light source 4.

[0167]FIG. 12 is an illustration of one configuration example of the gain control means 15 in the second embodiment of this invention. A circuit shown in FIG. 12 is made up of the input monitor PD 10, the output monitor PD 10 a and the gain control means 15. In FIG. 12, an input optical signal is detected by the PD 10, and then inputted as an electric signal to an amplifier 60 a of the gain control means 15 to be amplified in this amplifier 60 a, and further inputted as V_(i) to a divider 63. Likewise, in FIG. 12, an output optical signal is detected by the PD 10 a and inputted as an electric signal to an amplifier 60 b of the gain control means 15 to be amplified in this amplifier 60 b, and further inputted as V₀ to the divider 63. Additionally, in the divider 63, a calculation of V₀/V_(i) is conducted and a signal representative of the multiplication of that calculation result by a reference voltage V_(ref) is inverted and inputted to an amplifier 64. In the output of this amplifier 64, when the optical output is large (when V_(o)/V_(i) is larger than 1), a negative voltage appears. Accordingly, the output of the amplifier 64 is inputted to a transistor 64 a so that its base voltage decreases, which reduces the output of the excitation light source 4. The reverse operation is performed similarly. That is, the divider 63 performs the calculation of V_(o)/V_(i), and if the V_(o)/V_(i) is smaller than 1, tin the output of the amplifier 64, a positive voltage develops. Therefore, the output thereof is inputted to the transistor 64 a so that its base voltage increases, which increases the output of the excitation light source 4.

[0168] Thus, an operational flow in the wavelength-multiplexed light amplifying apparatus 41 shown in FIG. 11 is as follows. That is, an optical signal outputted from the fourth wavelength-multiplexed light amplifying basic unit 35 ((A) in FIG. 11)] is inputted to the wavelength-multiplexed light amplifying unit 30 ((B) in FIG. 11)] where it is branched into four. These four branched optical signals are inputted to the wavelength-multiplexed light amplifying units 30 indicated at (C) to (F) in FIG. 11, thus accomplishing distribution into 16. Likewise, although not illustrated in FIG. 11, an optical signal outputted from the optical amplifying section 2 b of the fourth wavelength-multiplexed light amplifying basic unit 35 also undergoes distribution into 16.

[0169] Furthermore, an optical signal outputted from the wavelength-multiplexed light amplifying unit 30 is inputted to the optical branching section 1 a in the optical feedback loop system 42, and an optical signal from this optical branching section 1 a is O/E-converted in the PD 10 a so that its power is inputted as output information to the gain control means 15. Furthermore, the optical power from the preceding unit is detected in the PD 10 and inputted to the gain control means 15. Still furthermore, on the basis of the output information and the detection result in the input monitor, a gain control signal is produced in the gain control means 15, and is supplied to the excitation light source 4 provided on the input side of the gain equalizer 13 to control the excitation condition of the excitation light source 4. Moreover, an optical signal from the preceding unit and an optical signal from the excitation light source 4 controlled are multiplexed in the multiplexer 9 and an optical signal obtained by the multiplexing is amplified optically in the EDFA 12 and gain-equalized in the gain equalizer 13. An optical signal outputted from this gain equalizer 13 is coupled with an optical signal form the auxiliary excitation light source 4 a in the optical branching section 1 a in the fourth wavelength-multiplexed light amplifying basic unit 35, and then outputted again. Incidentally, although omitted in FIG. 11, an optical signal outputted from the optical amplifying section 2 b in the fourth wavelength-multiplexed light amplifying basic unit 35 also undergoes distribution into 16, thereby realizing the broadcast function.

[0170] With this configuration, an optical signal from the fourth wavelength-multiplexed light amplifying basic unit 35 is broadcasted to be distributed into a desired number and the feedback control is implemented using the input monitor signal in the forefront unit and the output information in the rearmost unit, which enables appropriate wavelength-multiplexed light output control.

[0171] That is, in FIG. 11, an optical signal of 18 dBm inputted to the optical branching section 1 a in the fourth wavelength-multiplexed light amplifying basic unit 35 becomes 15 dBm at the output of the optical branching section 1 a, and additionally gets a gain of 3 dB in the optical amplifying section 2 a to be outputted at 18 dBm from the optical amplifying section 2 a. Furthermore, this optical signal of 18 dBm becomes 15 dBm because of branch into two in the optical branching section 1 a of the first wavelength-multiplexed light amplifying basic unit 31 in the wavelength-multiplexed light amplifying unit 30 indicated at (B), and further gets a gain of 3 dB in the optical amplifying section 2 a to again reach 18 dBm at the sections 1 a to 1 c are connected alternately, a resistance to the reflection develops, which eliminates the need for a configuration in which each of the optical amplifying sections 2 a to 2 f is interposed between optical isolators 3, so that the excess loss of the system including the optical amplifiers and distribution systems is reducible. Accordingly, regardless of no installation of the optical isolators, the stabilization condition of R₁·R₂·G²<<1 is satisfied, thus ensuring stable operation.

[0172] Still furthermore, since this wavelength-multiplexed light amplifying apparatus 41 utilizes a free port (one of two input port of each of the optical branching sections 1 a to 1 c) in the distribution systems, it is possible to eliminate the need for the multiplexer 9 which multiplexes the excitation light and signal light together, or to reduce the required number thereof.

[0173] Moreover, this wavelength-multiplexed light amplifying apparatus 41 can provide the expansion in the in-service. That is, after the introduction of this wavelength-multiplexed light amplifying apparatus 41, the branches according to the user's demand can be made very easily, and the expansion in the number of branches can be made easily without interrupting the current service: therefore, the service is improvable.

[0174] In addition, for example, if the unit (A) has a power sufficient to excite the following wavelength-multiplexed light amplifying units 30, the excitation light source 4 for the units (B) to (F) becomes unnecessary. Meanwhile, the excitation light source 4 can be installed for the unit (B) when needed, or the excitation light sources 4 can also be connected to all the wavelength-multiplexed light amplifying units 30 ((B) to (F))]. This signifies the flexible employment of the excitation method. Still additionally, it is also acceptable that one large-output excitation light source 4 is prepared and connected to the wavelength-multiplexed light amplifying units 30.

[0175] Furthermore, since, in this wavelength-multiplexed light amplifying apparatus 41, the optical amplifying sections (EDFA) 2 a to 2 f, the optical couplers 1 a to 1 c, the optical switches, the excitation light sources 4 and the optical isolators 3 constituting its optical components are designed to have the same characteristic, so that the wavelength-multiplexed light amplifying apparatus 41 is suitable for the mass production and has excellent expansion. For example, in FIG. 11, the units (B) to (F) except (A) are completely identical in configuration to each other, which enables the mass production.

[0176] Still furthermore, it can also be considered that the system is constructed by the wavelength-multiplexed light amplifying basic units 30 b shown in FIG. 6, in place of the wavelength-multiplexed light amplifying units 30 shown in FIG. 11. That is, this wavelength-multiplexed light amplifying apparatus 41 is made up of the plurality of wavelength-multiplexed light amplifying basic units 30 b connected in a multistage fashion and the optical feedback loop system 42.

[0177] Each of these wavelength-multiplexed light amplifying basic units 30 b is composed of the optical branching section 1 a having the two input ports P₁ and P₂ and the two output ports P₃ and P₄ for receiving a wavelength-multiplexed optical signal comprising optical signals of a plurality of wavelengths through the signal input port P₂ forming one of the two input ports P₁ and P₂ and for outputting the wavelength-multiplexed optical signal through the two output ports P₃ and P₄, and the optical amplifying sections 2 a and 2 b connected to the output ports P₃ and P₄ of the optical branching section 1 a, respectively. These wavelength-multiplexed light amplifying basic units 30 b are connected in a multistage fashion in a manner that the signal input port P₂ of the optical branching section 1 a is connected to the output side of the optical amplifying section 2 a or 2 b in the preceding wavelength-multiplexed light amplifying basic unit 30 b.

[0178] Yet furthermore, the optical feedback loop system 42 is made up of the excitation light source 4, the multiplexer 9 for multiplexing an excitation light from the excitation light source 4 and a wavelength-multiplexed optical signal together, the gain equalizer 13 put between the multiplexer 9 and the signal input port P₂ of the forefront unit 30 b (indicated at (A))] of the plurality of wavelength-multiplexed light amplifying basic units 30 b arranged in a multistage fashion, and the gain control means 15 for controlling the gains of the plurality of wavelength-multiplexed light amplifying basic units 30 b on the basis of output information from the rearmost unit 30 b of the plurality of wavelength-multiplexed light amplifying basic units 30 b arranged in the multistage fashion.

[0179] Moreover, the input monitor (PD 10) is provided to detect the input to the forefront unit 30 b of the plurality of wavelength-multiplexed light amplifying basic units 30 b disposed in the multistage fashion, and the gain control means 15 includes the control section 15 a which produces a gain control signal on the basis of the output information and the detection result by the input monitor (PD 10) and supplies the produced gain control signal to the excitation light source 4 placed on the input side of the gain equalizer 13 to control the excitation condition of the excitation light source 4.

[0180] Incidentally, it is also possible that the system is constructed using only one wavelength-multiplexed light amplifying unit, as well as the system according to the first embodiment described above with reference to FIG. 7.

[0181]FIG. 13 is an illustration of another wavelength-multiplexed light amplifying apparatus according to the second embodiment of this invention. A wavelength-multiplexed light amplifying apparatus 41 a shown in FIG. 13 is made up of a wavelength-multiplexed light amplifying basic unit 30 b and an optical feedback loop system 42. In this configuration, let it be considered that, as indicated by an broken line in FIG. 13, light outputted from a fourth wavelength-multiplexed light amplifying basic unit 35 in the optical feedback loop system 42 is inputted directly to an optical branching section 1 a in the optical feedback loop system 42.

[0182] In FIG. 13, the fourth wavelength-multiplexed light amplifying basic unit 35 in the optical feedback loop system 42 is provided with the optical branching section 1 a constructed as a 1:1 optical coupler and optical amplifying sections 2 a and 2 b having a gain equivalent to the branch loss in the optical branching section 1 a or having a gain lower than that branch loss. Additionally, an auxiliary excitation light source 4 a is connected to an input port P₁ of the optical branching section 1 a, while an optical isolator 3 is connected through an optical coupler 8, a multiplexer 9, an EDFA 12 and a gain equalizer 13 to an signal input port P₂ thereof. Incidentally, the optical branching section 1 a, the optical amplifying sections 2 a and 2 b and the connecting section between the optical branching section 1 a and the optical amplifying sections 2 a and 2 b in the fourth wavelength-multiplexed light amplifying basic unit 35 can also be constructed using an optical waveguide made of a glass or a semiconductor.

[0183] Furthermore, the optical feedback loop system 42 includes an optical branching section 1 a, an optical filter 100 a, a PD 10 a, a gain control means 15, a PD 10, an optical isolator 3, an optical coupler 8, an optical filter 100 b, an excitation light source 4, a multiplexer 9, an EDFA 12, a gain equalizer 13, an auxiliary excitation light source 4 a, a connector 5, and the fourth wavelength-multiplexed light amplifying basic unit 35. These are the as those described above, and the description thereof will be omitted for brevity. Additionally, the input to the fourth wavelength-multiplexed light amplifying basic unit 35 is detected by the PD 10, and the output optical power is detected by the PD 10 a and outputted as output information. Still furthermore, in the control section 15 a of the gain control means 15, a gain control signal is produced on the basis of the detection result and the output information and supplied to the excitation light source 4 placed on the input side of the gain equalizer 13 to control the excitation condition of the excitation light source 4.

[0184] Thus, this wavelength-multiplexed light amplifying apparatus 41 a includes the fourth wavelength-multiplexed light amplifying basic unit 35 composed of the optical branching section 1 a having two input ports P₁ and P₂ and two output ports P₃ and P₄ for receiving a wavelength-multiplexed optical signal comprising optical signals of a plurality of wavelengths through the signal input port P₂ forming one of the two input ports P₁ and P₂ and for outputting the wavelength-multiplexed optical signal through the two output ports P₃ and P₄ and optical amplifying sections 2 a and 2 b connected to the two output ports P₃ and P₄ of the optical branching section 1 a, respectively, and further includes the excitation light source 4, the multiplexer 9 for multiplexing the excitation light from the excitation light source 4 and a wavelength-multiplexed optical signal together, the gain equalizer 13 put between the multiplexer 9 and the signal input port 2, in the fourth wavelength-multiplexed light amplifying basic unit 35, and the gain control means for controlling the gain of the fourth wavelength-multiplexed light amplifying basic unit 35 on the basis of the output signal from the fourth wavelength-multiplexed light amplifying basic unit 35.

[0185] With this configuration shown in FIG. 13, an optical signal from the fourth wavelength-multiplexed light amplifying basic unit 35 is, as indicated by an broken line, inputted directly to the optical branching section 1 a in the optical feedback loop 42 for proper control, while another amplifying unit can be connected through the connector 5 to the optical amplifying section 2 b of the fourth wavelength-multiplexed light amplifying basic unit 35 for user's extension.

[0186] In addition, in FIG. 13, apart from the case indicated by the broken line, in the case in which the optical signal from the fourth wavelength-multiplexed light amplifying basic unit 35 is inputted to the wavelength-multiplexed light amplifying basic unit 30 b, needless to say, in the wavelength-multiplexed light amplifying basic unit 30 b indicated at (B), the optical signal is distributed into two to be broadcasted, and the output optical signal from this wavelength-multiplexed light amplifying basic unit 30 b is O/E-converted and fed back as output information, thereby accomplishing proper wavelength-multiplexed light output control. Additionally, the branch expansion becomes possible through the use of connectors 5.

[0187] In this way, even using only the fourth wavelength-multiplexed light amplifying basic unit 35, an optical feedback loop system is constructible, thus offering simplified equipment. Furthermore, if the user purchases this wavelength-multiplexed light amplifying basic unit 30 b newly and additionally after the introduction of the wavelength-multiplexed light amplifying apparatus 41 a, it is possible to increase the number of branches easily without interrupting the current service.

[0188] The present invention is not limited to the above-described embodiments, but can be used in various ways.

[0189] It is also possible to use a semi-fixed optical attenuator in place of the variable attenuator 11 b, or to use connectors 6 a and 6 b in place of the connectors 5 in the second embodiment.

[0190] In addition, owing to the broadcast function, this invention is also applicable to a distribution system such as an optical subscriber system.

[0191]FIG. 14 is a block diagram schematically showing an optical cross-connect system to which this invention is applicable. In FIG. 14, an optical cross-connect system 50 is a large-capacity optical switch applicable to a parallel super computer, an ATM core switch and others, and is made up of a broadcast section 51 for distributing a wavelength-multiplexed signal and a space switch/wavelength selector section 52. The wavelength-multiplexed light amplifying apparatus (see FIG. 1) according to this invention is applicable to a wavelength-multiplexed signal distributing function in the broadcast section 51 and in the space switch/wavelength selector section 52.

[0192] More specifically, this invention relates to a wavelength-multiplexed light amplifying apparatus applicable to an m×n matrix optical switch (switching) in the optical cross-connect system 50, and this wavelength-multiplexed light amplifying apparatus 31 is constructed using a plurality of wavelength-multiplexed light amplifying basic units 30 b, where m and n represent natural numbers and, in this case, each is 356. Additionally, each of the wavelength-multiplexed light amplifying basic unit 30 b is composed of an optical branching sections 1 a and 1 b each having two input ports P₁ and P₂ and two output ports P₃ and P₄ for receiving a wavelength-multiplexed optical signal comprising optical signals of a plurality of wavelengths through the signal input port P₂ forming one of the two input ports P₁ and P₂ to output the wavelength-multiplexed optical signal through the two output ports P₃ and P₄, and optical amplifying sections 2 a to 2 f connected to the two output ports P₃ and P₄ of the optical branching sections 1 a and 1 b.

[0193] Furthermore, these wavelength-multiplexed light amplifying basic units 30 b are connected in a multistage fashion in a manner that the signal port P₂ of its optical branching section 1 a is connected to the output side of the optical amplifying section 2 a or 2 c in the preceding wavelength-multiplexed light amplifying basic unit 30 b, and an excitation light source 4 is connected to the other input port P₁ of the optical branching section 1 a of the forefront unit 35 of the plurality of wavelength-multiplexed light amplifying basic units 30 b.

[0194] Thus, this system is applicable to a distribution system such as an optical subscriber system, and the expansion on the in-service is easily feasible.

[0195] Moreover, although the description of the first embodiment has been made above in the case in which this wavelength-multiplexed light amplifying apparatus is applied to branch ports (see FIG. 2), if other parts are used, it is also possible to construct an optical add-and-drop apparatus using this wavelength-multiplexed light amplifying apparatus .

[0196]FIG. 15 is a block diagram schematically showing an optical add-and-drop apparatus (optical ADM system Optical Add-Drop Multiplexer) to which this invention is applied. In FIG. 15, an optical add-and-drop apparatus 53 using wavelength-multiplexed light amplifying basic units is for dropping an optical signal with an arbitrary wavelength from a wavelength-multiplexed signal or for adding an optical signal with an arbitrary wavelength, and is made up of an optical feedback loop system 34 (left-side part in the illustration), a first broadcast unit 54 a comprising five types of wavelength-multiplexed light amplifying units 30 denoted at (B), (C), (D), (E) and (F), a second broadcast unit 54 b comprising three types of wavelength-multiplexed light amplifying units 30 denoted at (G), (H) and (I), an optical carrier signal outputting section 56, and a selecting section 55. In this case, the number of users in the first broadcast unit 54 a is 16, while the number of drops in the second broadcast unit 54 b is 8.

[0197] An optical signal, for example, in which 64 wavelengths (ch1 to ch64) are multiplexed, from the left side in FIG. 15 is inputted to the second broadcast unit 54 b where ch1 to ch32 are dropped for a maximum of 16 users, while other ch65 to ch96 from the optical carrier signal outputting section 56 are added thereto so that an optical signal in which 64 wavelengths (ch33 to ch96) are multiplexed is transmitted toward the right side in FIG. 15.

[0198] In FIG. 15, the optical feedback loop system 34 has a first-stage unit 65 for broadcasting a wavelength-multiplexed optical signal comprising optical signals of X kinds of wavelengths (X represents a natural number not less than 2).

[0199] Furthermore, the first broadcast unit 54 a is connected to the output side of the first-stage unit 65 to broadcast a wavelength-multiplexed optical signal outputted from the first-stage unit 65, while the second broadcast unit 54 b is connected to the output side of the first-stage unit 65 to broadcast a wavelength-multiplexed optical signal outputted from the first-stage unit 65 and to drop an optical signal. Furthermore, the optical carrier signal outputting section 56 is for outputting an optical carrier signal to add an optical signal, while the selecting section 55 is connected to the output side of the first broadcast unit 54 a and further to the output side of the optical carrier signal outputting section 56 to selectively output a wavelength-multiplexed optical signal from the first broadcast unit 54 a and an optical carrier signal from the optical carrier signal outputting section 56.

[0200] Furthermore, the aforesaid first-stage unit 65 is constructed as a first-stage wavelength-multiplexed light amplifying basic unit 75 including an optical branching section 1 a having two input ports P₁ and P₂ and two output ports P₃ and P₄ for receiving a wavelength-multiplexed optical signal comprising optical signals of X kinds of wavelengths to output the wavelength-multiplexed optical signal through the two output ports P₃ and P₄, with the other signal input port P₁, being connected to an auxiliary excitation light source 4 a (or an excitation light source 4), and further including optical amplifying sections 2 a and 2 b respectively connected to the two output ports P₃ and P₄ of the optical branching section 1 a. Accordingly, the first-stage unit 65 signifies a special example of the first-stage wavelength-multiplexed light amplifying basic unit 75.

[0201] Still furthermore, the first broadcast unit 54 a has five types of wavelength-multiplexed light amplifying basic units 30 (denoted at (B), (C), (D), (E) and (F))], and in each of the wavelength-multiplexed light amplifying basic units 30, the optical branching sections and the optical amplifying sections are connected alternately to each other, for example, in the order of the optical branching section 1 a, the optical amplifying section 2 a, the optical branching section 1 b and optical amplifying section 2 c. Thus, the first broadcast unit 54 a has a plurality of multiplexing wavelength-multiplexed light amplifying basic units each having the same configuration as that of the first-stage wavelength-multiplexed light amplifying basic unit 75, and these multiplexing wavelength-multiplexed light amplifying basic units are connected in a multistage fashion in a manner that the signal input port P₂ of the optical branching section 1 a is connected to the output side 2 c (2 d, 2 e, 2 f) of the optical amplifying section in the preceding multiplexing wavelength-multiplexed light amplifying basic unit. Furthermore, the first broadcast unit 54 a is constructed such that one input port P₂ of the optical branching section 1 a in the forefront unit of the plurality of multiplexing wavelength-multiplexed light amplifying basic units is connected to the output port of one optical amplifying section 2 a in the first-stage wavelength-multiplexed light amplifying basic unit 75. Incidentally, in this instance, although the number of users is 16, the expansion of the wavelength-multiplexed light amplifying unit 30 allows accommodation of more users in the in-service.

[0202] On the other hand, the second broadcast unit 54 b has three kinds of wavelength-multiplexed light amplifying basic units 30 (denoted at (G), (H) and (I))] to accommodate right users, and further permits additional connection of two wavelength-multiplexed light amplifying basic units 30 to achieve the expansion up to 16 users. Additionally, as well as the first broadcast unit 54 a, in each of the wavelength-multiplexed light amplifying basic units 30, the optical branching sections and the optical amplifying sections are connected alternately to each other, for example, in the order of the optical branching section 1 a, the optical amplifying section 2 a, the optical branching section 1 b and the optical amplifying section 2 c. Accordingly, this second broadcast unit 54 b has a plurality of distribution wavelength-multiplexed light amplifying basic units each having the same configuration as that of the first-stage wavelength-multiplexed light amplifying basic unit 75, and these distribution wavelength-multiplexed light amplifying basic units are connected in a multistage fashion in a manner that the signal input port P₂ of the optical branching section 1 a is connected to the output side of the optical amplifying section 2 c (2 d, 2 e, 2 f) in the preceding distribution wavelength-multiplexed light amplifying basic unit. Still furthermore, the second broadcast unit 54 b is constructed such that one input port P₂ of the optical branching section 1 a in the forefront unit of the plurality of distribution wavelength-multiplexed light basic units is connected to the output port of the other optical amplifying section 2 b in the first-stage wavelength-multiplexed light amplifying basic unit 75, and each of a plurality of drop optical filters 23′ for selecting and outputting an optical signal with a predetermined wavelength is provided on the output side of the optical amplifying section 2 c (2 d, 2 e, 2 f) in the rearmost unit of the plurality of distribution wavelength-multiplexed light amplifying basic units.

[0203] In addition, in FIG. 15, the optical carrier signal outputting section 56 is for outputting an optical carrier signal with a predetermined wavelength, and this function is exhibited by an optical E/O converter 56 a. Additionally, the optical E/O converter 56 a is controlled so as to prevent an optical signal with the same wavelength as that of an optical signal to be allowed to pass from being outputted.

[0204] Furthermore, the selecting section 55 is composed of Y optical switches (switch sections) 55 a, Y coupling sections 55 b, a multiplexing section 55 c and an amplifier 55 e, where Y represents a natural number not less than 2. Each of the Y optical switches 55 a is connected to the output side of the optical amplifying section 2 c (2 d, 2 e, 2 f) of the rearmost distribution wavelength-multiplexed light amplifying unit in the first broadcast unit 54 a. Additionally, the Y coupling sections 55 a are for coupling the wavelength-multiplexed optical signal lines of the optical switches 55 a with the optical carrier signal lines from the optical carrier signal outputting section 56. For example, this function is exhibited by optical couplers. Furthermore, in the coupling sections 55 b, optical signals outputted from two directions are coupled with each other. Still additionally, the multiplexing section 55 c is for selecting and outputting only one wave form Y kinds of optical signals from the coupling sections 55 b, with this function being exhibited by an AWG. Furthermore, one wave of the wavelength-multiplexed optical signal is selected by the number of the inputting port of this AWG, and optical signals with different wavelengths are multiplexed at every port and outputted. The amplifier 55 e is for amplifying a wavelength-multiplexed optical signal. The Y optical switches 55 a selectively conduct the switching between the operation of allowing Y kinds of wavelength-multiplexed optical signals from the first broadcast unit 54 a to pass so that they are outputted from the multiplexing section 55 c and the operation of outputting Y kinds of optical carrier signals from the optical carrier signal outputting section 56 through the multiplexing section 55 c.

[0205] The wavelength-multiplexed light amplifying unit 30 and the optical feedback loop system 34 are the same as those described above, the description thereof will be omitted for simplicity.

[0206] Thus, the flow of an optical signal in the optical add-and-drop apparatus 53 using the wavelength-multiplexed light amplifying basic units shown in FIG. 15 is as follows. That is, a wavelength-multiplexed optical signal from the left side in FIG. 15 is inputted to the first-stage unit 65 after passing through the connector 5, the isolator 3, the optical couplers 8, 8′, the multiplexer 9, the EDFA 12 and the gain equalizer 13 in the optical feedback loop system 34. Furthermore, in this first-stage unit 65, the output of the optical amplifier 2 a is inputted to the first broadcast unit 54 a so that an optical signal(s) with 64 wavelengths multiplexed is broadcasted.

[0207] On the other hand, the output of the optical amplifier 2 b of the first-stage unit 65 is inputted to the second broadcast unit 54 b so that an optical signal(s) with 64 wavelengths multiplexed is broadcasted, and a 64-wavelength-multiplexed (ch1 to ch64) optical signal appears at each of the output ports of the optical amplifying section 2 c (2 d, 2 e, 2 f) in the rearmost distribution wavelength-multiplexed light amplifying basic unit in the second broadcast unit 54 b. Furthermore, an optical signal with a desired wavelength is taken out from this 64 wavelength-multiplexed (ch1 to ch64) optical signal in the drop optical filter 23′, where the drop function is exhibited. Accordingly, the drop function of this second broadcast unit 54 b is equivalent to the drop function displayed with the cooperation of the AOTF 20 b and the branching section 21 in FIG. 2, and the drop optical filter 23′ (see FIG. 15) corresponds to the tunable filter 23 in FIG. 2. Incidentally, this drop optical filter 23′ can also be constructed using a wavelength-fixed optical filter.

[0208] Furthermore, in FIG. 15, the multiplexing section 55 receives an optical carrier signal from the optical carrier signal outputting section 56 and a wavelength-multiplexed optical signal from the first broadcast unit 54 a so that the wavelength-multiplexed optical signal from the first broadcast unit 54 a passes when the optical switch 55 a is in the ON state, and Y kinds of optical signals are inputted to the coupling section 55 b, where Y represents a natural number below X at the input. Additionally, in the multiplexing section 55 c, an optical signal with λ1 of λ1 to λ16 is selected at a first port, an optical signal with λ2 of λ1 to λ16 is selected at a second port, and similarly an optical signal with λ16 of λ1 to λ16 is selected at a 16th port. Still additionally, the outputs with λ1 to λ16 are again multiplexed, and then amplified in the amplifier 55 e and outputted from the right side in FIG. 15. On the other hand, when the optical switch 55 a is in the OFF state, the wavelength-multiplexed optical signal from the first broadcast unit 54 a is shut off, and the optical carrier signals from the optical carrier signal outputting section 56 are inputted to the coupling section 55 b, and are multiplexed in the multiplexing section 55 c and then outputted as a wavelength-multiplexed optical signal therefrom. This wavelength-multiplexed optical signal is finally outputted from the right side in FIG. 15 after being amplified in the amplifier 55 e.

[0209] With this configuration, the dropping operation is conducted in the second broadcast unit 54 b of the optical add-and-drop apparatus 53 using the wavelength-multiplexed light amplifying basic units while the adding operation is performed in the multiplexing section 55 c, that is, the add and drop function can be exhibited using the same wavelength-multiplexed light amplifying unit 30. Accordingly, the optical network is flexibly expandable and extensible using the features of this wavelength-multiplexed light amplifying apparatus.

[0210] In addition, the optical add-and-drop apparatus can employ another type.

[0211] For realizing an optical add-and-drop apparatus, a technical difficulty is that signal light is added to a wavelength dropped in that apparatus. That is, for the realization of the optical add-and-drop apparatus, in that apparatus, a desired wavelength must be sufficiently removed from wavelength-multiplexed signal light arranged at as an extremely narrow interval as, for example, 0.4 nm (n: nano, signifying 10⁻⁹), and another signal must be newly added to that wavelength.

[0212] In this case, technically difficult points are the following 1) and 2):

[0213] 1) to remove a specific wavelength without signals having other wavelengths being affected by the wavelength dropping; and

[0214] 2) in the case in which signal light is added newly to that wavelength, coherent crosstalk (intermodulation distortion) occurs between the added wavelength and the signal light left without removed.

[0215] Accordingly, one possible measure against these difficulties would be to make development based upon the use of an electrical area type add-and-drop apparatus. Furthermore, this measure enables effective utilization of wavelength slots (or bands). On the other hand, this I measure does not consider advantages/disadvantages or strong points/weak points of the add-and-drop in an optical area.

[0216]FIG. 17 is a block diagram showing a further optical add-and-drop apparatus to which this invention is applicable. In FIG. 17, an optical add-and-drop apparatus 53 a considers the aforesaid advantages/disadvantages and further realizes an optical add-and-drop function easily. The optical add-and-drop apparatus 53 a shown in FIG. 17 is made up of an optical feedback loop system 42 a, a first-stage unit 65 a, a first broadcast unit 54 d, an additional signal outputting section 68 and a transmission outputting section 69.

[0217] The first-stage unit 65 a has two input ports P₁ and P₂ and two output ports P₃ and P₄, and when a wavelength-multiplexed optical signal comprising optical signals with X kinds of wavelengths is inputted through a signal input port forming one of the input ports P₁ and P₂, a first wavelength-multiplexed optical signal is outputted from each of the output ports P₃ and P₄.

[0218] Furthermore, the first-stage unit 65 a is for broadcasting the first wavelength-multiplexed optical signal comprising optical signals with X kinds of wavelengths, and includes an optical branching section 1 a and an optical amplifying sections 2 a and 2 b, thus functioning as a first-stage wavelength-multiplexed light amplifying basic unit. In this optical branching section 1 a, its signal input port (input port P₁) is connected to an excitation light source 4 a, while its output ports P₃ and P₄ are connected to the optical amplifying sections 2 a and 2 b, respectively.

[0219] The optical feedback loop system 42 a has a first dispersion compensator (DC) 67 a in addition to the components described above. This first dispersion compensator 67 a is connected to an EDFA (amplifying section) 12 and further to a signal input port of the optical branching section 1 a in the first-stage unit 65 a. Still additionally, the EDFA (amplifying section) 12 for amplifying the optical signal to output an amplified optical signal is placed on the input side of the first dispersion compensator 67 a, and the first dispersion compensator 67 a compensates for the dispersion of the amplified optical signal and puts a compensated optical signal subjected to the dispersion compensation in the signal input port of the optical branching section 1 a.

[0220] The first broadcast unit 54 d is connected to the output side of the first-stage unit 65 a for broadcasting the first wavelength-multiplexed optical signal outputted from the first-stage unit 65 a, and has five kinds of branching wavelength-multiplexed light amplifying basic units 30, designated at (B), (C), (D), (E) and (F) and identical in configuration to the first-stage wavelength-multiplexed light amplifying basic unit 75 a. In each of these branching wavelength-multiplexed light amplifying basic units 30, the signal input port (input port P₁, P₂ ) of its optical branching section 1 a (1 b, 1 c) is connected to the output side of the optical amplifying section 2 a (2 b, 2 c) in each of the preceding branching wavelength-multiplexed light amplifying basic units 30, thus establishing a multistage connection therebetween.

[0221] In addition, the output port P₃ of the optical amplifying section 1 a in the first-stage wavelength-multiplexed light amplifying basic unit 75 a is connected to one input port P₁ of the optical branching section 1 a in the forefront basic unit 30(B) of the plurality of branching wavelength-multiplexed light amplifying basic units 30. Still additionally, a plurality of dropping filters 23 a each for selecting and outputting an optical signal with a predetermined wavelength are provided at the output sides of the optical amplifying sections 2 c (2 d, 2 e, 2 f) in the rearmost branching wavelength-multiplexed light amplifying basic units 30, designated at (C), (D), (E) and (F), of the aforesaid plurality of branching wavelength-multiplexed light amplifying basic units 30. Moreover, each of these plurality of dropping optical filters 23 a is designed to be capable of setting a wavelength to be dropped variably. Incidentally, the plurality of dropping optical filters 23 a can also employ a fixed-wavelength filter.

[0222] Accordingly, this first broadcast unit 54 d copies a plurality of wavelength-multiplexed optical signals for wavelength dropping (a drop section) and uses these copied wavelength-multiplexed optical signals; hence, the optical add-and-drop apparatus 53 a can devote itself to the extraction processing for a predetermined wavelength without paying attention to other wavelengths.

[0223] In addition, the configuration of this optical add-and-drop apparatus 53 a is not made to perform the adding to a dropped wavelength, and does not accomplish the effective utilization of the wavelength slot (or band). This is because an optical apparatus is capable of realizing a wavelength slot showing a sufficient degree (for example, 128 waves, 240 waves) without conducting such operations.

[0224] The additional signal outputting section 68 is for multiplexing a plurality of optical carrier signals to output a second wavelength-multiplexed optical signal, and is composed of a plurality of optical carrier signal generating sections (E/O) 56 a, optical amplifiers 20 a, optical band pass filters 68 a and a multiplexing section (WDM) 55 c.

[0225] The plurality of optical carrier signal generating sections 56 a are for outputting a plurality of optical carrier signals respectively. The optical amplifiers 20 a are for amplifying and outputting the plurality of optical carrier signals. The optical band pass filters 68 a are connected to the plurality of optical carrier signal generating sections 56 a, respectively, for conducting the filtering of the optical carrier signals outputted from the optical amplifies 20 a. Incidentally, since these optical amplifiers 20 a and optical and pass filters 68 a are provided for signal amplification, if the signal power is sufficient, the installation thereof becomes unnecessary.

[0226] Furthermore, the multiplexing section 55 c is connected to the side of the plurality of optical carrier signal generating sections 56 a for multiplexing the plurality of optical carrier signals from the plurality of optical carrier signal generating sections 56 a to output a second wavelength-multiplexed optical signal. For example, a waveguide type diffraction grating (AWG) can a used therefor.

[0227] The transmission outputting section 69 is connected to the output side of the first-stage unit 65 a and the output side of the additional signal outputting section 68 for coupling the first wavelength-multiplexed optical signal from the first-stage unit 65 a with the second wavelength-multiplexed optical signal from the additional signal outputting section 68. The transmission outputting section 69 is composed of a coupling section (CPL) 55 b, an amplifier 55 e and a second dispersion compensator 67 b. The coupling section 55 b is connected to the first-stage unit 65 a and further to the additional signal outputting section 68 for coupling the first wavelength-multiplexed optical signal from the first-stage unit 65 a with the second wavelength-multiplexed optical signal from the additional signal outputting section 68 to output a third wavelength-multiplexed optical signal. The amplifier 55 e is for amplifying the third wavelength-multiplexed optical signal from the coupling section 55 b and for outputting the amplified third wavelength-multiplexed optical signal, that is, it is for amplifying a wavelength-multiplexed optical signal. The second dispersion compensator (DC) 67 b is connected to the optical amplifier 55 e of the transmission outputting section 69 for compensating for the dispersion of the amplified third wavelength-multiplexed optical signal to put a compensation optical signal after the dispersion compensation.

[0228] This configuration performs the add-and-drop processing. In this case, in terms of the number of wavelengths, for example, the number of input wavelengths is 32 waves. Of 32 waves, first to 28th waves are allocated to “drop” and “through”, while 29th to 32th waves are allocated to “add”.

[0229] First, a wavelength-multiplexed optical signal (λ₁ to λ₃₂) is inputted from the left side of FIG. 17 and amplified in the first-stage unit 65 a, and subsequently a first wavelength multiplexed optical signal (λ₁ to λ₂₈) is inputted to the dropping first broadcast unit 54 d and the wavelength multiplexed optical signal (λ₁ to λ₂₈) is inputted to the through transmission outputting section 69.

[0230] Furthermore, the copies of a plurality of wavelength-multiplexed signals are made for the wavelength dropping in the first broadcast unit 54 b, and desired wavelengths (λ₁ to λ₂₈) are dropped in the dropping optical filters 23 a. Unlike the configuration shown in FIG. 15, the adding processing is not conducted in connection with the dropped wavelengths.

[0231] On the other hand, the second wavelength-multiplexed optical signal (λ₂₉ to λ₃₂) to be added is inputted from the additional signal outputting section 68 to the transmission outputting section 69. Additionally, the coupling section 55 b of the transmission outputting section 68 couples the first wavelength-multiplexed optical signal (λ₁ to λ₂₈) with the second wavelength-multiplexed optical signal (λ₂₉ to λ₃₂) from the additional signal outputting section 68 so that a third wavelength-multiplexed optical signal (λ₁ to λ₃₂) is outputted therefrom and is then amplified in the optical amplifier 55 e and outputted to the next relay point.

[0232] Thus, in the optical add-and-drop apparatus 53 a, when a predetermined wavelength is dropped and signal light is added, a desired wavelength can be sufficiently eliminated or dropped from a wavelength-multiplexed signal light arranged at an extremely small interval and another signal is added newly to that wavelength before transmission, accordingly exhibiting an add-and-drop function.

[0233] In consequence, the following two points are realizable. That is:

[0234] 1) Since the copy of a wavelength-multiplexed signal is used, a desired wavelength can be dropped in inconsideration of the influence on the other wavelengths.

[0235] 2) Even in the case in which signal light is added newly to that wavelength, no coherent crosstalk occurs between the signal light with a specific wavelength to be added and the signal light left without removed.

[0236] In addition, since the copy of a wavelength-multiplexed signal can be made in this way, it is possible to offer services to many terminal users.

[0237] Incidentally, this method is applicable also in the case of 64 wavelengths, 128 wavelengths and 240 wavelengths, and the number of wavelengths to be dropped and the number of wavelengths to be added are variable. For instance, in the case of 128 wavelengths, of these, 1st to 96th wavelengths are allocated to “drop” and “through” while 97th to 128th wavelengths are assigned to “add”.

[0238] The present invention is also applicable to another application in which an optical signal is distributed according to wavelengths. Additionally, this invention covers all changes and modifications of the embodiments herein which do not constitute departures from the spirit and scope of the invention. 

What is claimed is:
 1. An optical amplifier, comprising: an optical filter provided on an input side of a monitoring unit, said monitoring unit monitoring an optical signal, to allow a passing of a signal light component as well as remove an excitation light component from the optical signal.
 2. An optical amplifier, comprising: an optical filter provided between an optical coupler, said optical coupler performing an optical coupling, and a detecting unit, said detecting unit is for detecting an input light, to allow a passing of a signal light component as well as remove an excitation light component from the input light.
 3. A wavelength-multiplexed light amplifying apparatus, comprising: an input monitoring unit, said input monitoring unit including an optical filter which allows a passing of a signal light component from an input light as well as removes an excitation light component from the input light. 